This time, it is almost definitely a fatty tumor.
Totally harmless, except that they can grow to weigh 5 pounds, irreversibly stretching and deforming the muscle around them. Imagine a fatty tumor, 12% of your total body weight, just chillin' on your body. Now, imagine it attached to your rib.
Say it with me now: eeeeeeeeeeeeeeeeeeeeeeeeeew.
Poor little Nanna went through surgery this morning, and the vet says she did very well. I'll pick her up this afternoon and spend the evening holding a cold compress to her armpit, singing Beach Boys songs to her and thanking my lucky stars that I have such a good dog. I am so in love with her.