ehh you probably didn't gain 5 lbs of fat.. lots of water with the junk you ate as well.
When I'm extremely cut and my body is used to a strict diet.. you wouldn't believe what a bit of pizza and cake will do to my body within a day. It looks so bloated you can bearly see my abs. Its gross. But it settles down the day or two after.
You should put some serious focus on lifting.. to build muscle. Then you can eat that crap (not saying all the time.. because it makes you feel icky in general) and not even gain an ounce. Actually it does feed the muscles.. but thats a long speal on bodybuilding and bulking and cutting and blah blah blah blah.
You see what I eat in one sitting at times.. hahah *pig*