A small amount of bleeding
after sex can be a very normal thing. There are 3 places that blood can come from when you notice it in your vagina after sex: 1. the vaginal wall itself, 2. the uterus, or 3. the cervix. The vaginal wall is normally well lubricated during sex. If it is not, or if penetration occurs before there is adequate lubrication, this can cause small abrasions during sex resulting in bleeding. If your intercourse is especially painful, then I would suspect that this may have been your problem.
Another possibility is that the blood is coming from your uterus. If you recently had sex just before or after your menstrual cycle, this could be where the blood came from. The most likely source is your cervix. The cervix has a high concentration of blood vessels that can easily bleed sometimes with some trauma that occurs with sex.
While cervical cancer is a cause of bleeding after sex, it is rare today because of the success of PAP smears being done to help catch cases of cervical cancer early.
Please schedule an appointment with your OBGYN. He or she can perform a pelvic exam to make sure that nothing abnormal is going on in your vagina, and that this blood is not the result of any serious issue. Good luck.