Why do I always get sick during my runs?

I am in excellent shape and i run often. The only problem is that I get very sick during my runs. Like nauseated. And it doesn't take long to make this happen. Usually I'll be on the first mile and it starts bugging me. This would be understandable if I were really out of shape, but i'm not. This is just a constant issue... I am in very good health otherwise, and I have talked to medical people about it, but I have had very mixed answers that don't get me anywhere. Please, If anyone can offer me any advice or thoughts on the matter, it would be greatly appreciated.