Hi & Welcome. Since your girl friend had sex for the first time, the post coital bleeding could be due to breaking of the hymen during the coitus. Hymen is a piece of skin that is very thin which covers the vagina entrance. Hymen rupture can cause such bleeding if she has not torn her hymen earlier. However, nowadays with so many sporting activities in which women are involved the hymen frequently breaks down even before the intercourse. This hymen can be torn during vigorous exercises or horse riding or sometimes minor trauma also and sometimes it is so rigid that it does not tear while haviing intercourse. So the probability of rupture of the hymen is there. The other causes are inflammation or swelling and infection of the vagina or cervix, vaginal infections, cervical polyps, fibroids & endometriosis. It is very difficult to precisely confirm a diagnosis without examination and investigations. For an accurate diagnosis, she is advised to consult a gynecologist. I sincerely hope that helps. Take care