I have a j pouch, because i had ulcerative colitis, but now i'm worried that i might have crohn's disease, can that happen?

Yes, sometimes: Sometimes after colectomy, pouchitis can act like crohn's disease even if you don't have crohn's. Also sometimes crohn's colitis can act just like ulcerative colitis but not extend beyond the colon until after the colon is removed. It is often very difficult to distinguish between the two. Scope with biopsies can help. Ask your gastroenterologist about evaluation and treatment options.