What Causes Vaginal Bleeding During Sexual Intercourse?

Recently I had intercourse with my husband and on 2 occasions noticed bleeding during intercourse. I am 59 yrs old and postmenopausal. I was thinking it may be fibroids bleeding. The bleeding stops when intercourse stops. There is no pain. During menopause my doctor performed ultrasounds to check my uterus and cervix since I had spotting or short-term bleeding about every 3-6 mos. What do you think could be going on?