Diagnosed with ulcerative colitis, but now i'm worried that i might have crohn's disease, can't that happen?

Unlikely: Cd can affect any portion of the GI tract from mouth to anus, uc only affects the rectum and large intestine. Cd can extend through the full thickness of the tract lining - creating holes, infection, or connections to adjacent organs, uc is limited to the lining of the colon. Differentiating can be tough when the rectum and colon are affected by cd. A gastroenterologist should be able to help.