Cramps???

Hi everyone.. I'm currently on birth control and this has never happened before. I'm in the middle of my three month pack and for the past few days i've been having what feels like seriously painful menstrual cramps during the middle of the night. I have no idea what's going on and nothing relieves it, it just goes away after maybe half an hour. I don't know if this is related, but my bf came inside a few weeks ago and I took plan B just to make sure I didn't het pregnant. Does anyone know what this could be???