Why do i feel so nauseous ALL THE TIME?

I feel nauseous all the time! This has been going on for at least a month. I am positive I am not pregnant.Just saying, I am on birth control and have never missed a period. I feel sick to my stomach. Dairy makes it worse so i thought I was just lactose intolerant but it persists all the time without very much relief. No medicine works and I can't throw up I just dry heave but I have a constant feeling that I am going to throw up. I am a college student with lots of work to do and classes to attend and I can't pay attention cause all my body can think about is not throwing up in class. I need some relief I am 20 years old I am on birth control and that is it. anyone know anything to help me???? I can give more details if you can help.