Yesterday morning I discovered two things: Flower had been bitten again overnight, and she had, at some point between then and the last cleaning day (when I do my in-depth checks of each mouse), gotten a massive tumor. I took her up to the vets today, and they confirmed that it was a tumor, and gave me three options: attempt to operate (risky, potentially expensive), leave it, or put to sleep.
Based on the size and location of the tumor, I'd already decided that if it was confirmed to be a tumor by the vet, that I was going to get her pts - if it was a small lump on her leg or back of neck, for example, I might have opted to leave until it got bigger. So, she got put to sleep today. Poor thing.