I had unprotected sex on the day of ovulation what could be the reason sigh slight cramps & bleeding the day after ovulation?

Vaginal bleeding: With unprotected sex, prostaglandins in sperm could cause the cramping you have experienced. Have you had your pap smear? If not, you need to as bleeding during intercourse may indicate a problem with your cervix.

Mittleschmerz: Impossible to say for sure, but probably nothing unusual. Pain around ovulation is common and called mittleschmerz. It's definitely not implantation bleeding; implantation doesn't occur until 6 days after fertilization.