Complex Mass on Ovary

I have had chronic pelvic pain for about a year now and had a miscarraig in November.  Since then I have had episodes of lower back pain and pelvic pain; on and of for the past 2 months.  In this past month I had my period but it never stopped.  I went to my ob/gyn and the gave me progesterone to stop it and THAT didn't work.  My blood and urine came up negative for pregnancy but upon pelvic exam my Dr found that my uterus was enlarged (to the size of an 8th week pregnancy).  Ultrasound showed 2 fibroids and a complex mass (5.8cm) on my ovary.  NOW I have to go get an MRI...should I be worried.  I have read a few of the posting and it sounds like the u/s should have ruled out a cyst.  If it is not a cyst, what could it be?  Is it a sign of Ovarian Cancer.  No one in my family has had it but MOST of the women in my family have had HUGE fibroids and either had to get partial or full hysterectomys.  Should my assumption be that since I have to get an MRI this means the mass is definitely solid?