DownDillie- I had the exact same issues recently, got an urgent referral and the scan was normal. I also have IBS and get the pains you mention.
Your mind can run riot and make you feel that you have every symptom you have found out from DR GOOGLE. The elevated ca125 levels could be caused by fibroids. ( I had them but didn't know until my scan )
Here's the interesting thing and may explain about the spotting and staining. Utrogestan is VERY UNSTABLE in the body. It can work for years and then start to be a bit erratic. It's the nature of the beast, and can lead to the bleeding you've mentioned. A professor of gynaecology told me this recently as I couldn't understand what was going on.
Hope your scan goes well. Try not to worry, you may not even need a biopsy. Let us know how you get on x