I'm a 20 year old female, about 5"9 and only weigh 100 pounds, I am at least 20 lbs under weight but I eat a lot and I eat normal meals.

Underweight . Try to see u pcp for evaluation and work up.
Go slow. First make sure that you get checked by your medical doctor to make sure that you don't have an obvious cause for the low weight like a hyperactive thyroid. Next concentrate on eating larger portions but make sure that the food is actually good for you. Please don't turn to the high calorie junk foods because at some point the weight will come in and it will be hard to give up the junk food.