Tired of being skinny

I'm 5'4 and I weigh 115, which is 5 pounds less than I was around this time last year.

When I went to the doctor earlier this month and I saw my weight, I was so upset that I weighed 115 and felt so discouraged, as if I would never be able to gain weight. Sometimes I feel like I'm doomed because both of my grandmother's are small, so genetics are playing a major role here. Anyways, can anyone give me some advice on how to gain weight?

I work out, but that will stop once I go back to school (because of my schedule) and I have a fast metabolism.

Am I underweight for someone who is turning 20?
Tired of being skinny
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