weight gain

bnm
I feel like I should need to gain a little more weight. I'm almost 19 and I'm only 4'11 but I stick at a constraint 90 pounds. I don't gain weight! I can lose a few pounds..last month I was at 84.... I don't know how to do this in a healthy way?? I eat and drink tons just never in my life been able to gain. The most I have ever wieghed was 94-95 pounds.