I sometimes gain a pound or two after eating strictly for a few days, I don't know why it happens, either, but then I'll check the scale a couple days later and the weight has come off. I do know that those little extra bites of high-fat or higher calorie foods do add up. I know you mentioned peanut butter, and when I eat that, I take the tiniest bit out of the jar with a knife, it's probably about a teaspoon. It's super-easy to eat too much of it, so that might be one of your problems.
I wouldn't worry too much about it, the weight loss will happen! This sounds kind of icky and embarrassing
but sometimes I'll weigh myself after having had a large bowel movement and I lost a pound or two! So it could be a anything.