Hi Becky. I had tonsil cancer, stage 4a, and finished my treatment ( cisplatin and 7 weeks IMRT) in October, in January I had a fairly invasive biopsy of several sites inside my throat (under general anasthesia), shortly after that I had a LOT of mouth pain and tooth/gum sensitivity to temperature and sweets. The ent told me it was a combination of trauma from the procedure with a topping of thrush. It eventually went away after 2 weeks of diflucan, along with painkillers and Magic Mouthwash ( my formula had Nystatin in it).
To help control the thrush, I use Biotene toothpaste and mouthwash for drymouth (because drymouth promotes thrush) and I eat active culture yogurt and take acidophilus supplements which fight the organism that causes thrush.
We're looking forward to your clean PET scan too, nothing better than a clean scan.
Best wishes ( and fingers crossed for you)
Mike