Gaining weight in a healthy way

I’m 5’2” 107 pounds. I used to be 125 a while ago now. But I don’t know why or how I lost so much weight. I don’t do or take anything except the birth control pill. I eat like a fat ass pig. So I’m just confused. I made an appointment to meet with my primary care doctor cause I told her about this, and she wanted to see me. But I canceled the appointment the day before cause I’m scared.

So does anyone have any healthy ways to gain weight. Anything to take? Like I read Boost is supposed to help and Ensure. But never tried it yet.