What Causes Cervical Bleeding After Intercourse?

Hi, I'm 19 yrs old. I am 5'5 and weigh 110 pounds. I am experiencing cervical bleeding after intercourse when I go to the bathroom afterwords. I am also experiencing alot of pain during intercourse. I have been to my doctor once already and got a Pap. It can back negative for everything. She told me that if the bleeding kept up that I should call her APAP. I have called her, and the soonest I can get into see her is Sept. Please help me! Thank You.