What Causes Bleeding After Sex Without Pain?

I'm 40 yr old with 2 teenage kids. When I have sexual intercourse I usually bleed with no pain at all. I was on pills a year ago. I was seen by gaenycologist and had transvaginal ultrasound done which was normal. Internal examination as well was normal. The doctor told me that this could be due to thinning of my uterus from long term oral contraception use. My pap smear was also normal. Why is this bleeding happens on most occasions?