How to gain weight on less than 800 calories a day

Ugh...feeling frustrated at the moment. I'm a week and a half out from the surgery, and although I dropped 10 pounds in the first 5 days after surgery because of the clear liquid post-op diet, I haven't lost anything since. I was OK with that....but this morning I got on the scale and I've gained 2.5 of it back!!!

I'm trying to figure out how that is even possible??? I'm tracking every calorie that I put into my mouth on sparkpeople and it shows that I'm taking in an average of around 700 - 800 calories per day. So, eating SO little, ESPECIALLY compared to pre-band, how on earth is it possible to GAIN weight?? I don't understand my body. Obviously I can't give up bc this post-op diet is required by my surgeon. But I sure do feel like failure at the moment. And, this kind of thing only makes those nagging voices stronger - the ones that are constantly asking me if I REALLY think this is actually going to work for me. At this point, it is very easy to doubt my self.