Underweight. Need Help.

Rosie • static.rose ❤️

I'm 5 ft 1 in and 91 lbs. I keep trying to gain weight and gain nothing and most of the time I lose weight. Advice? I'm concerned about my fertility when my BMI is 16.9. I need serious help. My GI is looking into Crohn's since it runs in my family, and I'm severely constipated. I have a lot of issues.