Oops posted this on one that replied to you, new to this site, oh well.
Has your doctor ever tested you for your thyroid gland? I had a friend who was 5.7 and she weighed less than 90lbs, its not because she didn't eat, she ate lots, but just couldn't gain weight. They figured out it was her thyroid gland and they put her on medication, I haven't seen her in a long time but last time I saw her the medication was helping. You should look into it.