What Causes Vaginal Spotting After Sexual Intercourse?

hi I am a 30 yr old female with 2 children.. I went to my gyno for slight spotting after intercourse, my doc performed a pap, sonohystogram and tranv vag ultrasound, and all came back normal.. The only thing found was a small spot of cervical erosion.. could this be an early sign of cervical cancer, or a misdiagnosis?? How does erosion occur?? All of my paps have been normal since age 18, and no history of this illness in my family..