What Causes Bleeding During Sex?

I bleed profusely during sex, but it stops when the sex stops. This has only happened since I started having sex with my current partner and his penis is very large - larger than any man I've ever had sex with. Could size have anything to do with it? I have regular gynecological exams - all paps are negative. I do however, experience some bleeding during the exam. My doctor says that my cervix is soft and therefore, sensitive. I haven't been back to my gynecologist since I've started having sex with my current partner. I am not currently using any birth control. I am 47 yrs old, 4 feet 11 inches tall and I weigh 111 lbs. I have one child, a 21 year old girl and she has been my only pregnancy.