Working out really hard will make me feel sick too if I do it for too long. In fact, I think it's pretty common. When I was in Track & Field in high school, some students would push themselves so hard during practice they would puke. It still happens to me quite a bit, especially if I do an intense cardio workout and then do some intense weightlifting afterwards. I will get through 1 set and need to stop because I feel like puking. It's just your body's way of telling you to stop (my opinion). Your body needs to get used to the level of activity try to dial down the intensity level until your body gets more used to exercising.