It doesn't happen all the time, but sometimes when my bf and I have sex, I will start to orgasm and before I peak it becomes painful cramps in my lower stomache (I think more on my lower left side). Sometimes the cramps will last for 10 mins or longer and I feel like I can't move. Usually after this happens I will start bleeding lightly the next day and it eventually actually starts my period. The blood is much darker than my menstrual cycle normally is though- sometimes almost black. I usually bleed for a week or so and the pain can remain vague for a day or two. There are no other symptoms.
Do you know what this could be a sign of?