Sending Rosie lots of love and healing thoughts to her, I had the same experience with Louis a couple of months back, a lump appeared on his side out of the blue, it grew quickly and the vet said it could be a number of things so did a needle biopsy from the result he said Louis needed an operation to have it cut out, we were so worried and the result was a benign tumour
Fingers crossed that Rosie's will be nothing to worry about