A more thorough evaluation with an exam and a sonogram is needed to determine the cause more precisely.
I have to assume that you have had abnormal Pap smears since you are getting them done every 6 months. There are several possible reasons why a woman can bleed after their Pap smear. First - it may due to the abnormal cells themselves. If a woman has HPV (human papilloma virus) in her cervical cells and HPV has caused some abnormal changes in the cells, they can bleed due to the disordered growth from the HPV and if they are progressing into precancerous cells (not common, but possible) then the cells bleed easily with Pap smears, or during or after sex. Secondly, and also very common, the brush used to scrape the cells from the cervix is abrasive and can cause the sensitive delicate inner cervical cells (the gland cells) to bleed just from the Pap brush. A third possible cause is the presence of a yeast infection or vaginitis, or sexually transmitted infections, all of which cause inflammation of the cells and result in them bleeding easily.
Hope that helps!