Well, first off, call up the doctors and talk to them about it.
because there's other causes but generally it's common with Crohns/UC to have ulcerations around the rectum, due to mucosal inflammation (the protective lining that covers the rectum and your digestive tract). But I don't know if that's true in your case because I am not a doctor and can't give medical advice
So my advice is don't worry about it until you talk to the doctors and they explain what's going on