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.