What Causes Constipated Feelings After Unprotected Intercourse?

We're not trying to conceive right now, but my boyfriend has been ejaculating inside of me when we have sex. I'm supposed to get my period anywhere from the 27th of the end of the month to the first 5 days of the next month. I think I'm getting my period, but I keep feeling the need to stay over the toilet like I need to push. What's going on?