Can't gain weight?

Okay so I'm gonna preface this by saying that I used to weigh almost 200 pounds. In the past year I've lost 50ish pounds, and I'm at a healthy weight for my height. I did restrictive diets to lose weight, but at this point I'm working out so if I gain weight I don't really care because I know I won't gain weight in my stomach.

There's just one problem. In September, I had my gallbladder removed. I'm not supposed to be eating oily food, but I'm a teen so of course I'm going to. The first month after my surgery, I ate like a pig and I didn't exercise. I lost 10 pounds. I have bulimia, but I'm in recovery and I don't purge often at all (like only 1-2 times a month when I relapse). At this point, I literally can't stop eating. I'll be full as hell but I still can't stop. I've recorded it, and I've been eating about 3500-4000 calories a day for the past few weeks (according to my Fitbit, I only burn about 2200). I haven't gained a pound. All I've done is lost (1-2 pounds a week). I should also mention that I have super low blood pressure (the doctors don't know why; I eat salty stuff all the time). Every time I stand up, I get lightheaded and spotty vision and I have to lean against something. I don't know what to do. Any medical professional that I talk to about this says that I'm "just a teen with a high metabolism." But I've had to lose weight in the past so I don't understand.