How do you if you have ulcerative colitis or just the stomach flu?

No stomach flu. There is no "stomach flu." this is a common misconception, unfortunately perpetuated by some physicians. Influenza is flu. It's a serious illness that kills up to 40, 000 people in this country every year. Stomach viruses, or gastroenteritis, are illnesses that involve vomiting and diarrhea, sometimes fever etc... Stomach viruses go away. Ulcerative colitis doesn't. That's how you know.
Ulcerative colitis. Ulcerative colitis can be relatively easy to diagnose because it normally affects only the colon and rectum and usually causes an obvious change in daily bowel habits, such as frequent stools containing blood or mucus. Your doctor will conduct a medical history and physical exam before doing other tests.