Postcoital bleeding

I have postcoital vaginal bleeding. I'ts not a strong bleed, it starts with a few drops and usually it's increases a bit and bleed for a several days after. I've already had a PAP test done, twice in the last two years, it was negative. The last one was about 8-10 months ago. No one in my immediate family had cervix or uterus cancer.
I'm 32 years old, i'm usually healthy. I have two kids, both in a vaginal bearth. I never had this problem before my first child was born. I had him when I was 27 years old. My period is quite regular most of the time. Can you please help me solve this mystery? How can I fix it??? I work in a medical library so I have have access to a lot of information, a reference would be nice.