Same here, even though it feels awful it's a good idea to stay on it so that you don't get worse. If your colonoscopy doesn't show any problems then it ought to be safe to start tapering afterwards. The colonoscopy will tell you a lot about how your insides are doing, they might find you still have inflammation or narrowing and increase your prednisone or switch out some of your meds and you should have better luck with having your symptoms improve
I got super bloated whenever I ate or drank anything while on pred (still kinda do 3 weeks off of it) so it could be any combination of the two causes that make your stomach puff up too. In case you are like me, you might notice that the more fluids you drink the more swollen and puffy you get all over the place from the pred causing water retention.
When was the last time you saw the GI? If he checked out the bloating and swelling and said you're ok, you should be alright unless it starts to get worse for some reason, then you should tell him right away. If you haven't recently been checked out it might be a good idea to have him check you out again if you wouldn't see him otherwise till the end of the month.