Why am I underweight?

21 year old male. 5 feet 9 in. tall. and 120 lbs. I eat well and still can't seem to put on any weight. Also, I notice that when I go for a few hours with any food, I get nauseated and sick. It seems like I am always hungry and in need of food so I do eat a lot, yet never gain weight. Any suggestions? I don't burn a large amount of calories either. Thanks for any answers