Great advice given. The only time I felt that way is that I drank a protein shake too close to my workout time, or I ate my snack too close to my workout. The only other time I've seen that is on the BL when they pushed the contestants really hard. Hope you find what the issue is soon
@SQUIRRELLYONE said it really well. It isn't normal to get an upset stomach after working out. Okay this is not a glamorous topic, but another possibility is that it's gas/bowels. With the deep breathing and muscle tension, it's working your digestive system. A light snack before working out is all that needed. The bulk of your energy actually comes from the meals you consumed the previous day.
How much are you eating before you exercise? If you go in with having eaten nothing or too much, you might feel a bit ill. If you're feeling like you're going to throw up, you're likely working out at too high an intensity and should work your way up to that intensity.
Remember to warm up and cool down. Try eating a yogurt about an hour before you work out, and make sure you're drinking water afterward. Good luck!