@Apocalyptichorsewoman
Just caught up with your post. Takes me back to when I was in your position. A faint in the middle of root canal ended up with referral to Gynae, scans and blood tests. (Via a trip too A&E when dentist called 999 but the junior doctor managed to miss the 2 kilo tumour which was taking up most of my abdomen. Tbf, I'd missed it, too. Thought that it was the natural consequence of being lazy about pulling my stomach in.)
First, don't worry about the Ca125. It's really not v reliable for diagnostic purposes as it can be raised for all sorts of other things and also some women with ovca have normal scores. Mine was 130.
But there's a tick-box index and, along with your age and what sounds like a 'complex' cyst, it qualifies you for more investigations. Which is good, but scary. I got an MRI, too, and later a biopsy, because everything was coming up 'inconclusive' and, v luckily, the gyne dept at my local hospital was being overseen by the experts at St Thomas'.
An MRI is much more detailed than an U/S. (I sympathise, because I nearly fainted in the middle of that, too. Had to press the alarm.) And more expensive. So the likelihood is they don't know yet whether it's cancer. It's the pathologist who has the last word on that.
In my case, I was transferred to the experts, had everything gyne removed along with the tumour and diagnosed with a Borderline, 'cured by surgery'. It was a monitoring scan for that, 5 years later, that picked up my colon cancer.
Now I've emphasised your positives, because I know that's what I'd have wanted to hear at your stage, but, sadly, Bridget and Liz had different results. You just can't tell. I held on fast to my GP's comment, 'It's a v good thing that you're feeling so well'.
They'll want to take it out, whatever it turns out to be. Make sure it's a gyneoncologist who does the surgery. Everyone says it really does make a difference.
And I'll be thinking of you tomorrow. 
And a carrot for the horse.