Ok so I am 4'11 and the same, about 92lbs, but I do have quite a bit of muscle if that makes a difference?  It is a normal BMI, perfectly healthy honestly. BUT... I'm not sure why you don't have periods so your dr would know more about that. I don't think you need to gain weight personally. You're not "too skinny" in my opinion.  Now... if you ask me... I don't have periods either, not for a long time, but it's not because I'm underweight it's because of The Pill + End Stage Kidney Disease.  Are you on any type of Birth Control? That can change things.