Is it possible to call the vet and get his/her opinion? I really think that would be best since they already know her history. I know what you mean about her being terrified. Peanut has always been that way about the vet, and is even more so since her surgery last year. I keep her confused with the carrier because sometimes all we do is go in the motorhome - which is no big deal to her - but other times we go to the vet and that terrifes her.
I really think calling the vet would be the best idea. They may give you thinks to look/feel for, or they may want you to bring her in right away to get ahead of the tumor before it gets too bad.
Good luck and here's hoping it will just go away on it's own again like it did last time.
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