As others have mentioned, your gain is probably mostly water weight - you have to eat 3500 calories to gain a pound of actual fat. BUT you may have thrown yourself out of ketosis, which is going to affect your losses. The best thing you can do is to get back on the horse and look forward.
I want to address something though - you are a month into the program and planned a cheat. This is a no-cheat diet for a lot of reasons, not the least of which is that we need to learn to make better food choices in ALL situations, not just the everyday. I have been on IP since the end of July 2012, and we have had quite a few wonderful dinners out - I stayed 100% OP for every single one. I went to my 20th HS reunion and stayed OP. I got through Thanskgiving and Christmas 100% OP. And I am not unique.
It is not only POSSIBLE to have "Special" times without going off plan, it's NECESSARY. Ultimately you have to weigh the pros and the cons of any cheat, but I have always felt that I didn't want to spend one extra minute on Phase 1 than absolutely necessary. If I had cheated along the way, I wouldn't be in Phase 2 right now - and I am VERY glad to be there