This may be your problem. You've been low on protein for quite a while and now that your body is getting enough, it's using what it's getting to make much needed repairs and rebuild muscle that you've been missing. Stick with it. Once your body is satisfied that it's gotten enough to do what it needs to, the weight should start coming off more steadily. While 1,300 calories does seem a bit low, I'd say keep doing what you're doing but to try not to go below 1,200; a body that is stressed from too few calories is not going to let go of the extra fat easily.