Very freaked out - No fibroids, so what can cause an enlarged uterus?

I am 55 years old, last period was 2 years ago, I am about 80 pounds overweight but have never had any health problems. PAP smear normal. During routine physical last week, I had a pelvic ultrasound and transvaginal ultrasound. My doctor called and wants me to have a biopsy because my uterus is a little large (15 cm) and the tests did not show fibroids. (I had some fibroids 2 years ago during last ultrasound)  She said I shouldn't be concerned but wants to rule out cancer. That word concerns me! I have to wait 2 weeks for the biopsy and I'm going nuts! I've read that hormonal imbalance may cause and enlarged uterus. I have NO symptoms at all - no bleeding, no pain, no bloating, I feel good. Any comments would be very much appreciated. Thank you