Listen to your body. Make sure you get your fluids in. For an upset stomach, try clear fluids first (eg water, weak tea, broth, gelatin, fruit juice diluted 1:1 with water) and then go to mild foods (eg. toast, cereal, soup, yogurt, milk if you can tolerate it). Most healthy people are just fine with a few days like that but if it goes on for longer than a week or so you probably want to check with your doctor.
Hope you feel better soon!