Weight gain problems

I know this is something that you almost never hear, But I’m a girl who needs to GAIN weight. I was born with an over-active metabolism. its so fast that its near impossible for me to gain weight, and easy to loose. But once i loose it i cant gain it back for years! I’m so close to being underweight again its scary. I’m 5’6″ tall, 22 years old, and i only weigh 106lbs. (I lost two pounds again recently wile I’ve been trying to gain.) (I should be according to my height, age, and frame, between 120-133lbs)

Last time i actually dipped down into the danger zone was about seven or eight years ago. I got sick and couldn’t eat for a week. I dropped down from 109lbs to 85lbs. I just recently got that all back, but now its going down again. and I’m constantly eating. i even out eat my man.

anyone know how i can at least maintain the pounds i currently have? My doctor advised me to not take weight gain pills unless i was pregnant, (which i am not and dont plan on being anytime soon) as they can mess me up further.