In addition, with deep penetration, the cervix can be bumped, causing cramping with expulsion of some left-over
menstrual blood (or early blood if your
period is about to arrive). The cramping may be due to this, or it could be that you're ovulating. In mid-cycle during ovulation, some women experience some mild cramping from time to time.
Again, if this continues, see your gynecologist. And be sure to use lots of lubrication and don't have p-v sex until you're very turned on.