When I was 5'3, I was only 90 lbs. Now I'm 5'8, and 115 lbs. I eat like a horse, and don't gain weight. It's called a fast metabolism. Yes, you are underweight. But as long as you're eating, there's not much you can do about it.
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If you hadn't used poor grammar, I wouldn't have needed to correct it.
I am not at fault.