It's usually nothing to worry about, but you should mention it to your ob-gyn just in case. Sometimes it can be a sign of a problem.

Usually, it's just a sign that you're not lubricated enough. Many women bleed after manual sex or vaginal sex if they are not lubricated enough. The inside of the vagina is so fragile it can tear from being too dry. This can cause quite a bit of bleeding. It's also the most common cause of pain during sex.

If you find that you are not getting lubricated enough on your own, and the doctor says nothing more serious is causing the bleeding, try some store bought lubrication. Water-based is usually the best.