Sometimes During Sex With My Boyfriend, I Begin To Feel

Sometimes during sex with my boyfriend, I begin to feel very nauseous and get an intense cramping feeling in my stomach. I feel like I need to have a bowel movement and then I feel like I need to vomit. My skin goes very pale and clammy.Usually after a few moments of feeling like this I vomit and then I feel better after I've been sick. Any ideas on what this could mean?