I had my period a week ago and now after sex i'm having mucous-ey vaginal bleeding. What could cause this?

I've been having irregular, pop up after nowhere periods since i started my first year at college last fall. I just started having sex a couple of months ago. My first 3 times were really spaced apart and i bled during each time and i was a little sore- i figured this was just normal first-time-having-sex bleeding since it never continued to bleed. After the third time, I was having sex pretty regularly- about 4 times a week or so. My boyfriend has only ever been with one other girl and they were both virgins when they had sex for their first and only time, so I don't think I have an STD- unless I contracted it from something else. My last period ended a week ago, and I had sex the next day after it ending and had no problems. Four days later I had sex again and I bled and continued to bleed for the next 2 days. It was very light and mucous-ey blood. And I decided to go to the doctor since I've been having irregular periods in general and i figured it could just be a wonky period. But before going to the dr. and after I had stopped bleeding for those 2 days we decided to experiment. We wanted to see if the bleeding was caused by sex- so we started having sex and he used his right hand to finger me before any penetration and his left hand after actually having sex. There was only blood on his left hand. And I continued to bleed really lightly after and the next day. I have no other symptoms than this light bleeding. Does anyone have any idea what this could be??? I'm really concerned. And since my mother insists on going to the dr. with me and I cant let her know I'm having sex I would like to have some idea of what this could be before going. Please answer fast!!