Hi.
Since the uterus contracts during orgasm, it’s possible that a small amount of blood and tissue left over from your last menstrual cycle (or building up for your next cycle) is being expelled. Many women experience light spotting after orgasm. Die this bleeding happen after you had an orgasm? If so, there's your answer.
If not, it may be that your cervix was irritated, which can result in light bleeding. Short anatomy lesson: once you’re sufficiently aroused, the uterus rises up, moving the cervix out of the way of a penis so it doesn’t get bumped. If you’re having penis-vagina sex before you’re turned on, your cervix is vulnerable to bumping and irritation.
You may also be experiencing tearing and abrasions if you’re not turned on. A pelvic exam can determine the extent of the tissue trauma, if there is any. If p-v hurts, you need to tell your partner to ease up and slow down. Never, ever have p-v until you're sufficiently aroused. And be sure you’re wet enough—or get some good lube to augment your own natural lubrication. I recommend Replens, which is available at most drugstores.
It’s also true that bleeding can also be a sign of something serious like an infection or injury, so you might want to ask your gynecologist about this during your next exam. Good luck to you. Dr. J
This happened to me also, It's more common than you think.