How to gain more weight?

well, I guess I'm the opposite of too many people around. I've lost around 15 pounds in about 3weeks in the beginning of October, and I wanna get it back (and a bit more)

I'm 5'3ft and just 90 pounds... now, I'm eating as regular as I can but it took me a month to get back 5pounds (I was 85lbs in Oct.) and now I can't seem to be able to get more weight :/ (and I would like to get to 110-115lbs till January)

What can I do?

Does anyone here have this problem too?
How to gain more weight?
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