I have unexpected bleeding after sex, what is that?

Possibly infection: Common causes of postcoital bleeding include vaginal or cervical infections. Also, "rough sex", can cause this. A doctors visit can usually pinpoint and treat the cause.

Needs to be checked: Could be as simple as thin skin, pH imbalance or irritated glands, but could be more serious such as polyp, growth or abnormal cells that would need a biopsy. Have it checked.