Not stupid at all. I'd say it varies, sometimes up to a week or two depending on the intensity of the workouts. Yes, 2.5lbs is normal just like TOM. I can go up to 5lbs in water then finally a whoosh sometime later. Water will help with your bloatedness.
That's part of why I dislike starting out on a new regiment because I know there will be a temporary gain. Funny, I know the cause and etc. yet I still get upset if I see a "gain" on the scale. I'm ridiculous sometimes, haha.
not last long but as long as you know you're doing everything right, it will be all good. 100 calories extra is not going to make you gain, usually an excess of 3500 calories (give or take, depending on one's metabolism) will make you gain. Don't stress!!!