Post coital bleeding as it is called, needs to be investigated straightaway. It could be due to any of the local lesions including cervical polyps, cervical erosion and an unhealthy cervix
. You should see a gynecologist for a local examination, a pap smear
and a trans-vaginal sonogram as well to locate the cause. Meanwhile abstain from further intercourse, and if you miss a period, check for pregnancy
also. Wish you good health.