Low Blood Sugar Symptoms

No matter what I eat, I can?t gain weight. This has been a problem all my life. For the last five years, I have had the additional problem that if I don't eat as soon as I get hungry (which is about every two hours), I start getting sick. I first get stomach cramps. Then, after about 20 minutes, I start to get really hot and dizzy and feel like I'm going to throw up. I suspect that if I didn?t eat at that point, I would pass out. What could be causing these symptoms? I think my great aunt wasted away from this problem.