I Went To My Obgyn Today Due To Back Pain,

I went to my Obgyn today due to back pain, irregular bleeding, and cramping. He diagnosed me with endometritis and prescribed antibiotics. As a precaution he wanted to do a ultrasound, on the ultrasound they discovered an irregular shaped cyst on my ovary and ordered a ca125 test. I am concerned that this is cancer. A little info on me, I am 38 have been on lo lo Estrin fe for 15 months, I have a family history of breast cancer (mother, aunt, and 2 cousins) my doctor seems to think that I should not be concerned that it could be endometriosis, yet I am still concerned. Any advice would be helpful.