unexpected weight gain

So I'm a pretty skinny women and always have been but recently i have noticed sudden weght gain....I exercise at least 3 times a week for 15 20 minutes and eat fairly healthy have a pretty active life....is it possible pretty weight gain? I'm not dued for my period for 11 days. The birth control i have has no hormones in it to cause weight gain. I drink water every day...has anyone else experienced this. Also it can't be my age i am only 19 thanks anything will help...this is my body i went from 112 to 117