How can I slow my metabolism and gain weight?

I am 30 years old and have not gained a single pound since high school. I'm 5'1'' and weight exactly 100 lbs. If by any chance I gain an extra pound or two (I get super excited about it), I lose my appetite right away and the weight goes right off. I can easily shed pounds, but it's so difficult to gain weight. I've even tried drinking the weight gainer shakes from GNC, but no luck. I eat food real late at night, hoping my metabolism will be slower while I sleep. It seems like my body metabolizes my food too quickly. Things exit my body within 4-5 hours (sorry for the graphic description). Is that normal? What can I do to gain weight? I have trouble finding clothes and I'm tired of hearing people say "you need to gain weight". Please help. Your suggestions might help me try a new strategy to gain weight.