Weight loss after surgery? I can't keep weight on

I was already thin and couldn’t afford to lose any weight. I had my surgery a week ago. I look sickly. I’ve lost 13lbs in a week. Granted the first 2 days after surgery I didn’t eat much, but since then I’ve been eating everything in sight. My appetite is through the roof. I’ve been drinking weight gain drink mix too. I haven’t been this hungry since I was a teen.
I am at a loss. Does this happen after this kind of surgery? It’s not like I’m puking. I haven’t thrown up at all since surgery. Any ideas? I’m going to ask my Ns at my follow up on friday too.