What Causes Vaginal Bleeding During Sexual Intercourse?

I had not had sex in a year until a month ago. The first 5/6 times was normal no problem. Then I had intercourse last week and during saw very thick red blood. After wiping off, the blood went away and I saw no more but we did not continue the intercourse. The next night we attempted again and saw the same but not as thick and red, but it went away as soon as we stopped. What could this be? It's been a week know and we want to try again but don't want to see blood. I did a PAPs and ultrasound but could not see any problems. The doctor said I might have poped a blood vessel. What do you think?