So you're halfway through a superset when your stomach starts feeling... anything but super. You're working out, and you're nauseous as hell. What gives?

If you just guzzled an entire bottle of water, that may explain it. “Your stomach has a certain amount of acid in it to break down food, but if you have too much water in it at any one time, the acid actually floats above the water,” explains New York-based trainer Chris Ryan, C.S.C.S. Add to that jumping up and down, and that fishbowl of H2O sloshing around can leave you queasy.

Eating right before a workout might also make you feel sick, as well as working out on an empty stomach, says Brian St. Pierre, R.D., C.S.C.S., director of performance nutrition at Precision Nutrition. Ideally, you should eat a mixed meal two to three hours before a workout, or a small snack shortly before getting to the gym, he adds.

And if you had a high-fat meal before you worked out, that time frame for discomfort almost doubles, Ryan points out. "Fat takes over two times as long to break down as protein or carbs," he explains. "If you’re eating high-fat foods right before a workout, your digestion is going to be sluggish and can leave you feeling nauseated."

The questionable state of your stomach can also just be a sign that you’ve gone too hard, too quickly—which won’t surprise anyone who's been pushed to the point of puking by a hard-ass coach. “When you exercise, especially intensely, blood is taken away from GI tract and sent to the muscles working really hard in order to deliver oxygen,” St. Pierre explains. The blood draining from your stomach can induce nausea. This is exacerbated if you’re working out in the heat, where your body is so focused on keeping you cool that it’s also shuttling blood to your skin, Ryan adds.

Bottom line: Eat a light, low-fat meal two to three hours before a workout and try not to drink too much water beforehand. If you're feeling hungry, snack on something light and low-fat.