What Causes Vaginal Bleeding During Intercourse?

i am 31 yrs old and have a 3 yrs old child through normal delivery. Earlier this year in February I conceived another baby which was sid to be in the tube, so I had a d & c in april. I had taken the needed care and precautions. A month later after we resumed sex life, i experienced light bleeding every time we had sex. So again we abstained for a month. Then again I had few drops of blood the next time we had sex. So we are confused and worried. What should I do? What could be the reason? I dont have pain or anythign otherwise. Kindly help.