Why Was I Bleeding From The Wall Of My Vagina Post Sex?

I had Hystorectumy 20 years ago. I haven t had sex for about 3-4months and I am bleeding inside , I am. 52 it s like a creamy pink color , but I had a bad cramp went to the bathroom and when I wiped my self I noticed red blood , I went to a obgyn and she said I was bleeding from the wall in my vagine , and that was it ..I don t under stand , is it going to stop .? What and why is this happening