Why am I gaining such a large amount of weight so quickly?

I am eighteen years old and I am 5'4. I have been struggling with an eating disorder of some sort for almost 3 years. I was throwing up multiple times a day until about a week ago. At one point, I weighed about 115, but after I began throwing up (I couldn't always purge as soon as I needed to or I couldn't get all of it out) I gained about 10 pounds. But Saturday was the last time I purged and I weighed about 121. Now, I eat around 1000-1200 calories a day, I drink more water, and I exercise a bit more than I did while purging. But, I've gone from 121 to 129 since Sunday. How is this possible and how do I make it stop?