What have you been eating? And where are you in your monthly cycle?
If I eat a cheat meal, pretty much anything from restaurants, I get bloated and stay bloated for 3 or 4 days even. My water weight doesn't drop off the next day. Salt and excessive sugar are the big offenders for most people. So it's very possible that you're experiencing water weight that's just lasting a few days. Have you had a salty meal recently? That'll do it.
Also, your cycle can definitely cause you to retain water. Personally, I always hold onto at least 1lb of water weight for the whole week before my TOM, then that pound plus some drops off as soon as I start. Everyone's pattern is different though.
In order to gain 2lbs of fat, you'd need to eat 7,000 more calories over your maintenance limit. That's fairly significant. Quite possible if you had a blow out weekend or something, but if you've been staying on plan, that's highly unlikely.
If you're a daily weigher, I suggest you make notes when you record your weight. I personally have a notepad right by my scale. Most mornings I just record my weight, but I always record the day I start my TOM and I'll also record things like "salty cheat meal" or "went off plan, too many cookies" or something. That way I can go back and follow how these events affected my weight.
Success is a journey, not a destination
Goal Weight reached on: June 14, 2010
Monday Accountability Weigh-in: 136.2 - 10/10/2011...time to get back on the pony!