Swallowing..

So in the past I have always swallowed when giving my ex oral but it always made me feel so sick for the next couple days so I stopped and started having him just cum on me. Well we had broken up so I hadn't done it in a while but I am now with my current boyfriend and I gave him oral for the first time the other night and once again it's been 3 days and I still feel sick. I thought it was just with my ex but apparently not so does this happen to anyone else? Do you know why?