I don't understand what's causing the vaginal bleeding if i don't have any stds just started after sexually intercourse?

A few possibilities: If newly sexually active, maybe continued hymen tear or other local injury. Otherwise, bleeding after sex often means cervicitis, i.e. inflammation of the cervix. Gonorrhea, chlamydia, and trichomonas are common causes, but most cervicitis has unknown causes. Abnormal cell changes due to HPV is another possibility. Some of these are serious, so see your doctor; it should be easy to find the cause.