Cramps

Bl

I’ve had cramps since ovulation ( i know i already ovulated based on OPK and ultrasound. ) my opks are back to very light after a obvious positive and I’ve never had this before. It’s basically been one sided but today (4 dpo) it’s more in the uterus area/ my lower back? Almost like my period is about to start but I’m not due for another week. What could this be?