You didn't feel any pain during your long run, but it's been three days and your hamstrings are still really sore. Like, still hurts-as-you-sit-down-on-your-couch sore. You're starting to wonder if you may have actually hurt yourself and need to see a doctor.

"Muscles can be sore after a hard workout and tendons can become inflamed and tender with overuse, but these generally aren't things that require evaluation by a doctor," says Dr. Scott Rand, sports medicine doctor at Houston Methodist. "What we worry about are the sports injuries that can cause permanent damage if they're not evaluated and treated properly. Sometimes, these injuries are subtle. But, more often than not, these injuries are acute and come with sharp, noticeable pain."

Still, it may be tricky to decide when it's just soreness and when it's a serious injury — especially if the pain is in a muscle you use all day, every day, like a quadricep or a bicep.

How to decide if your pain is "just soreness"

The human body is a sophisticated, but complex, machine. You have more than 5,000 muscles, tendons, ligaments and bones, so it's no wonder that you experience pain in the ones that are most heavily used from time to time. So when is that pain "just soreness" and when is it not?

"In the days following a heavy workout, if you're experiencing tenderness and tightness but still have near normal strength and range of motion, it's likely just your everyday post-exercise soreness," says Dr. Rand. "It's annoying, but the good news is that this type of pain isn't worrisome and you can continue to exercise without concern."

(Related: Should You Still Work Out When You're Sore or in Pain?)

The most common cause of muscle soreness is called delayed onset muscle soreness, or DOMS. While using your muscles during a workout, they become slightly damaged. The harder the workout, the more muscle damage. But don't worry — this damage, typically, isn't a bad thing. In fact, it's part of the muscle-building process.

This exercise-induced muscle damage is, however, what leads to muscle soreness. (Related: Is Lactic Acid Buildup Really What Causes Muscle Soreness After a Workout?)

As muscle is damaged, very tiny microhemorrhages form and release blood. As it turns out, blood is very irritating to muscle, and this irritation leads to muscle spasms that ultimately cause muscle soreness. Typically, the tenderness and tightness associated with DOMS improve as you move and only lasts for a couple of days.

"DOMS is very common and isn't worrisome," says Dr. Rand. "However, it's possible for muscle damage to go beyond what's normal during a very intense workout. If your muscle is very thick and swollen after a heavy workout, it could be a sign of a serious condition called rhabdomyolysis — and you'll want to consider seeing a doctor."

Another common culprit of soreness is tendonitis, which is inflammation of a tendon. Tendonitis is a very common overuse injury that can happen anywhere in the body. In most cases, it can be treated with at-home remedies, such as rest, ice, compression and elevation (R.I.C.E.). (Related: Ice Vs. Heat: When to Use Which for Aches & Pain)

But, if your tendonitis is chronic and doesn't seem to be improving, it may require activity modification.

"At the end of the day, if your strength and range of motion seem normal, you can press on through the pain that comes along with muscle or tendon soreness," says Dr. Rand. "But, if you have localized pain that is sharp, limits your mobility, changes your gait, affects your range of motion or is accompanied by significant weakness, it could be a sign of a serious injury that needs treatment."

When pain warrants evaluation by a sports medicine doctor

Aside from the fact that being in pain, well...hurts, some sports injuries can can cause permanent damage if they're not evaluated and treated properly. This means that it's important to know when it's time to do something about your pain.

"Muscles can strain, ligaments can sprain and bones can break," Dr. Rand explains. "Most of the time, these are acute injuries that cause sharp pain, but, regardless of whether the pain comes on quick or slow, these types of injuries require evaluation and carefully planned treatment."

When it's a rupture, tear or break, it's probably pretty obvious that you need to see a doctor — but even muscle strains are best handled by a sports medicine doctor.

Seek help for a muscle strain

A muscle strain is when your muscle fibers pull apart and fray. If all you do is rest a muscle strain, Dr. Rand points out that it won't actually get any better.

"A muscle strain that goes untreated will stay chronically weak," explains Dr. Rand. "When we evaluate a muscle strain, we determine where the strain is exactly, how bad it is and what you'll need to do to heal — which generally involves resting until you're nearly pain-free, then performing exercises to regain your flexibility and finally working to regain your muscle strength."

Seek help for a stress fracture

Another type of pain that requires evaluation is the pain that can signal you have a stress fracture — a tiny crack in the surface of bone that forms as a result of overuse. At foot strike while running, if you feel a sharp pain in either your groin, the front of your shinbone or in your forefoot (behind your toes), it's time to see a doctor.

"Stress fractures are more commonly seen in women than men, and, if you're experiencing this type of pain, you should immediately stop activity and be evaluated by a doctor," warns Dr. Rand. "A stress fracture in any bone can cause pain, but if a stress fracture in one of these areas completes to become a break, it can limit your activity and mobility for a long time."

Seek help for growth-plate pain

Overuse injuries to an adolescent's growth plates can also cause pain, which sometimes warrants further examination. There are growth plates at the end of every long bone, as well as at the heel. Pains at these growth plates is pretty common, hence the term "growing pains." Unless pain at a growth plate causes your child to limp, comes with a lot of swelling or limits your child's mobility, it's generally not a huge concern.

"The growth plate pains we do worry about are when adolescents who are overhead throwers feel a sharp pain in the elbow or front of the shoulder at the first part of a throwing motion," says Dr. Rand. "That particular type of pain during that particular step of a throw can be a sign of a more serious injury to a growth plate, which can ultimately cause long-term damage and limit mobility if not addressed."

At the end of the day, it can be hard to determine if and when it's time to actually do something about your pain. As a good rule of thumb, Dr. Rand offers the following advice:

"When it comes to determining whether your pain is just soreness or something more serious, what I like to tell people is this: Pain that you earned and that goes away is typically nothing to worry about, but pain that you didn't earn and that affects your mobility warrants evaluation," Dr. Rand says. "If your strength and range of motion seem normal, you can press on through the pain that comes along with muscle or tendon soreness. But, if you have localized pain that is sharp, limits your mobility, changes your gait, affects your range of motion or is accompanied by significant weakness, it could be a sign of a serious injury that needs treatment."