Why do i keep losing weight instead of gaining it?

five foot two inches tall and 100 lbs (on a good day) i have had nausea since i was young, especially in the morning. till i found my trusty friend maryjane and now i have no problem eating mounds of food. Only one problem that to most wouldn't seem like a problem but a miracle I CANT GAIN WEIGHT. to my parents and my boyfriends dismay and to my own. i love food just cant keep it on. i use to weigh 145-150 when i was 12-16 now 100 at 21. thought i was suppose to start packing on those freshmen 15 but that never happened!!! HELP