My Boyfriend Says My Vagina Is At An Unusual Angle,

My boyfriend says my vagina is at an unusual angle, and he has trouble getting it in. I thought maybe I had a tipped uterus, but rear entry sex is enjoyable and so is deep penetration. We had been doing it with my knees over his shoulders, but now that gives me GERD. I'm 50, overweight but not obese, have never been pregnant, and have had sex without problems before. How can we find a position that is easier for penetration? I've Googled, but been unsuccessful so far.