Vaginal Tears/Fissures

For some reason unknown to me, the interior of my vagina gets these small tears either randomly out of the blue or after sex. Not even my gynecologist is able to diagnose the problem as all my bacteria levels, PH, yeast, and over all vaginal health is normal. It is as if my vaginal lining is so thin and fragile. My boyfriend of 4 years is a champ (he is normal sized). Regularly we go 1-3 months without sex while the tears heal as sex can be painful with the tears present. Any diagnosis, things to help, similar experiences?! Help!!