What Causes Vaginal Bleeding During Foreplay?

Hi, I want to know why I have vaginal bleeding during foreplay. It happened to me a week and a half ago. And then it happened again yesterday. In between that time I did not have sex. This has not happened to me before. I have been with the same partner for over six years now and I am certain it is not any form of STD, vaginal dryness as there was plenty of lubrication, or that my partner was being too rough.