I've had experiences like that, and it can be frustrating. In fact, a few times I've given up completely. It's like, if I gain weight on 1300 calories a day, then to heck with it. I don't want to have to eat 800 calories a day just to lose weight as a discernible rate.
Usually it just turns out to we water or whatever is in my digestive system, I think. A cup of water weighs a pound, and sometimes we hold onto a bit of extra water. Or other stuff.
Sometimes people say it might be muscle gain, but in my personal experience, I have to do a lot heavy (for me) weight lifting before I gain enough muscle to register on the scale. Some people seem to build muscle more quickly, though, so if you've been doing strength training, then perhaps.
If you want to tell us how much you weigh and how many calories you are eating, that might help.