As bad as this sounds...he might be TOO big...which you would think would be impossible right lol...but it's not. His penis could be hitting your cervix every time he goes in too deep, and it can cause you to feel nauseous. My friend was with this one guy for 3 years and thought it was normal to feel like puking, but then when they broke up and she ended up getting another boyfriend and said that it was so different because she didn't feel sick every time. So she ended up asking the doctor, and yea she was embarrassed, but that's pretty much what they told her.
That's not to say you should break up with him, but as awkward as it may seem, try talking to him and asking him if he can try and take it easy and not go as far in as he can and see if that helps.
So to all the guys who brag about how big they are, don't, you could be making your girl sick to the stomach lol.
Good Luck