Menstrual Cramps after Sex? Nausea?

Hey ladies, this has only happened to me since my new bf and he is very

well endowed. I often get terrible menstrual feeling cramps after we have sex. We use withdrawl so I dont think it's his semen that's causing it. I also got very nauseous after swallowing last night, and I normally don't. Any idea what could be the issue?