Blood is a kind of funny thing, a small quantity can colour the bowl...and be very, very impressive...not saying that you are not bleeding (and you are right to post and ask) but just that you need to monitor it closely...how much is a lot? Does it drip on its own or is it just mixed in with the stool? (I know, gross quesitons but important)....yes, you may bleed post take down but it should taper off slowly and not normally get worse. Add to that the pizza that may be somewhat acidic and cause irritation to the pouch and you could have your answer (did you drink some sort of red coloured drink? soda or gatoraide? That can dye the output too...I freak myself out every time that I eat veggie chips...the beets turn my output bright red!)
Keep a close watch on it (my surgeon used to make me collect my stool in a container (a sawed open milk jug or a very large jar with a lid (avoid anything with sharp edges or glass if squatting over it!) to get an accurate measure of what is really going on...you can do that too...and if needed take it in to ER with you if it gets worse...
Please let us know how you are doing...if you get dizzy, light headed, start having chills or fever then do not hesitate to call your surgeon or go strait to ER.
Until then stay off of the pizza, keep well hydrated and rest.
Sharon