If your stomach or side hurts when you run, it may have to do with dehydration or breathing technique. Image Credit: lzf/iStock/GettyImages

There's nothing worse than having your run interrupted by pains, indigestion or side-stitches that can ruin your pace — aka "runner's stomach." Even though running-related stomach pain is pretty common, it doesn't have to be your normal.

Here's how to figure out what could be the cause and what you can do to prevent it from happening again.

Causes of Stomach Pain When Running

"As unfair as it may seem, exercise in a small percent of people can cause gastroesophageal reflux disease (GERD)," says Niket Sonpal, MD, a gastroenterologist at Brookdale University Hospital and Medical Center in Brooklyn, New York.

GERD happens when the acid in your stomach chronically flows back up your esophagus. According to the Mayo Clinic, GERD is diagnosed based on how often and how severely you experience this acid reflux. To be diagnosed, you have to have it at least twice a week, or you need to experience severe acid reflux at least once a week.

"The underlying problem is that GERD is due to a weakened lower esophageal sphincter (LES)," says Dr. Sonpal. That's the muscle that regulates the flow between the esophagus and stomach. "Exercise can trigger heartburn if the LES muscle is weak or too relaxed and food or stomach acid 'sloshes' back up from your stomach into your esophagus," he says.

For this reason, Dr. Sonpal suggests you avoid exercising soon after eating. A good time frame is to allow at least one hour after you eat to exercise, says the Mayo Clinic, but even two to three hours may work better for you, depending on the meal and the type of exercise you do.

If you're worried about acid reflux or GERD and it is stopping you from exercising as normal, Dr. Sonpal suggests talking to your doctor. "See your doctor about medication and lifestyle changes you can make to keep active," he says. He adds that if you are overweight, weight loss can help improve GERD symptoms.

Read more: 12 Essential Tips for New Runners

What Causes Side Stitches?

Anyone can experience side stitches when exercising, and you've probably heard that everything from being out of shape, breathing incorrectly or being dehydrated can cause them. But according to Dr. Sonpal, the exact cause of side stitches is unknown.

For that reason, it's important to know that they can happen to anyone (no matter how "in shape" or not), says the American Medical Society of Sports Medicine. They are also harmless, so it's not a huge cause for concern. But side stitches are annoying and sometimes quite painful, so is there anything you can do about them?

If side stitches are really bothering you, ask a professional trainer or physical therapist to watch you while you run to see if there's anything form-related you could work on. Improving overall flexibility and core strength may be helpful as well. If you suspect a side stitch may be something more, always have a doctor evaluate you, just in case.

Preventing Running-Related Stomach Pain

"If you have reflux, then it's best to see a doctor so we can help with lifestyle changes and possible medications," says Dr. Sonpal. "When it comes to prevention, making sure that you are well enough to exercise is always paramount, not running on a full stomach and getting enough hydration can help with the majority of issues."

To stay hydrated, make sure you are getting the recommended amount of fluids every day. According to the Mayo Clinic, that's 15.5 cups (3.7 liters) of fluids for men and 11.5 cups (2.7 liters) for women. And, if you're exercising, increase the amount that you drink.

You also might consider working with a nutritionist or dietitian to evaluate your eating habits and see whether changing your diet might help with any digestive issues you're experiencing when you run.