I dont think you put on fat but if you gained weight its water, i am doing body opus, this friday i weighted 200 pounds in the morning and today monday i weight 206 pounds, thats 6 pounds after weekend carb up, but i still feel really loose in my jeans so its water.
If you havent cheated for 3 months it would take your body some time to react to the extra calories, i would say 2 weeks of cheating and you would put on some fat.
This is what happens when i bulk up for too long, first 2 weeks seem fine i put on weight but not much fat then after that watch out gut and buy some new pants.