help? I need to gain weight!

I'm a college freshman, and I've always been skinny- genetics and high metabolism and all. I eat like a horse, but for some reason I lost 10 pounds instead of gaining the famed "freshman 15." When I got home from winter break, I got the stomach flu and lost 5 pounds in 8 hours. I was so weak and exhausted, but I'm back up to 93 pounds, which is still low for me. My doctor said if I lose another pound, she'll put me in the hospital, but no matter what I do, I can't gain weight and keep it. Suggestions?