CA125 rising but PET Scans look clear.

Diagnosed with High Grade Ovarian Cancer, Stage 3, 4 1/2 years ago.
I'm 82 years old, now on my sixth round of Chemo--a repeat of single agent Carboplatin that I was initially on 4 1/2 years ago along with Taxol and major surgery.
I've now been on the Carbo for over a year with minimal side effects and excellent results. However, despite two clear PET Scans and no symptoms within the last few months, after a lot of cancer seen before I began the current regimen my CA 125 is rising. (Carbo was started when my CA 125 was over 1500 and I was having symptoms, had gone back to normal and now is 110. ) I'm not eligible for a lot of newer drugs since I have CKD likely from Avastin and I had to stop Elahere, which was working, after a few infusions as I had a severe pneumonitis.
My Oncologist is wondering whether the Cancer is not longer as responsive to the Carboplatin. I'm also on Procrit shots for my anemia.
I'm wondering whether how the CA125 rise should be interpreted. I switched to Procrit because I was told it was safer than the Transfusions I had several of first. My iron level was getting high, for example. Thank you for any input about new ideas. RZH