Back from the vet for Tethys' recheck after the surgery. She's healed up well, but there's another lump in about the same area that's growing fast. It's technically possible that it's an incision reaction, in which case it should subside on its own soon, but much more likely that it's a (more aggressive) tumor.
The vet seems fairly optimistic about her chances with a second surgery, but I expect that if it's coming on this aggressively this soon after surgery, there's probably more than just the one tumor. So I think we'll just be doing palliative care, when this gets big enough to bother her.