It is not uncommon to bleed a little bit some times. Especially if you are new to intercourse, with a new partner or trying a new position. If it happens every time, or does not stop, you need to speak to your doctor.
It can come from many places.
* You will almost always get very small micro tears during sex. These are just because of the movement and stretching of some very delicate tissue. They are so small that if does not cause any problems, and you do noteven realize that they are there. If it is rough sex or you were not wet enough, you can however get heavier bleeding from that.
* Sometimes his penis can hit your cervixduring deep thrusting and cause bruising and some light spotting or bleeding.
* Your cervix will open up during menstruation and ovulation. Other times it will be closed. After menstruation, some menstrual flow can stay behind when the cervix closes up and trap the left over blood. During sex, your uterus moves around, and an orgasm cause contractions to ripple through the uterus wall. This can force out this left over flow. It will normally be brouen, not red.
* Some women will spot during ovulation. Sex at that time can cause the blood to be expelled.
* Medicines like antibiotics and the pill can cause unexpected mid-cycle bleeding.
* Sometimes your hormones levels can be confused and you can get something called breakthrough bleeding.
* If you are a virgin, it can be your hymen tearing.
I hope you are having safe sex if you are not trying to make a baby.