Why do I bleed during deep penetration sex?

Deep sex. I can only think that there may be some excessive friction occurring that causes your mucosa to bleed. Another cause could be a vaginal lesion that gets irritated during sex, but that needs to be examined.
Cervical Irritation. There has to be a reason and this is usually an infection or irritation of the cervix that is being aggravated with intercourse. I would recommend a thorough gyn examination and testing to make sure that your infections are dealt with.