11 year old daughter has 5x7 mass on left ovary

[ReginaC]

I just needed to write or talk to someone because I am scared to death.  My daughter started her period when she was almost 10.  She has had a period just about every week since (which has been just a little over a year).  We were told that this wasn't uncommon to be irregular because of hormones at her age.  We finally went to a Gyn and they found out that she was anemic.  They thought the best way to treat this menstrual problem was with the birth control pill but, first she wanted to do an ultrasound.  She said it would be pretty much impossible to do a pelvic exam on my daughter.  
She did the ultrasound and said that my daughters left ovary measured 5cm x 7cm and that the normal size was 2cm x 3cm.  She said there was a mass on her left ovary.  She referred us to the hospital for another ultrasound.  I guess it will be more detailed.  
I am just not sure what to think right now. I don't know if there is a difference between a mass and a cyst.  You just automatically think the worst when you hear the word mass.  I am concerned that this could most likely be cancer due to her age and menstrual history.  Please help me with some information so I don't feel so left in the dark.  The doctor wants to wait until this next ultrasound before we jump the gun but, I can not help to worry.  Please help me and thank you so much for this site.                                                             ReginaC