Why do I get brown discharge/spotting??

When my bf and I make out and I get wet, I get cramping/blip feelings in my uterus. Does my uterus contract due to me getting wet? Are slight blips near my uterus normal when getting wet? I’m only asking because sometimes when I hang out with him and we make out, I get brown discharge. It’s always every other month and it’s exactly one week before my period each time (I’m also on the pill). I don’t know if I get the brown discharge because of my uterus slightly cramping. Anyone know why I get this brown discharge?