why do i cry while having sex with my boyfriend?

Me and my boyfriend have been in a relationship for about 8 months now but we've been together for about 10 months & occasionally when we have sex i just start bursting out crying 😳 it doesn't happen so much now but the first 5 months or so it happened atleast 1 out of ever 5 times we've had sex & it usually happens when he's hitting it from the back ... its not too rough or anything and the only way i could explain it to him is because i felt emotional 😭🤦🏽‍♀️ what do you think ?