What is an average/healthy weight for a 20 year old female?

I know im under average. I am a 20 year old female (im 5'2) and also short for my age lol. For a while i weighed 109 now i lost a few and down to 100.5 i know its not really healthy. i was wondering what the average weight is. my mom has said thyroid runs on both sides of my family so i may have to get that tested. she mentioned something about its most likely not it but mentioned an ulcer..not really sure what that is if anyone could tell me..thanks. also past 2 days my upper stomach has been hurting some sometime saround the time im about to eat. the one time i knew i wasnt going to throw up after i ate but it felt like i had to but wasnt going to. what are any good ways to gain some weight because right now i feel to skinny. someone said peanut er sandwhiches help. im not a fan of them but ill do anything to gain weight. I dunno if this will have anything to do with it but gall bladder also runs in my family they said its genetic almost everyone of my aunts and uncles and some cousins have had theirs removed. Any ideas....i dont tell my mom these things right away cause she tends to worry lol. THANKS.