Thank you all for your input - maybe within time the vet community can get a better idea of what causes cancer in these little guys...
I'm glad that her vet is not pushing her to get and vaccines.
I did stop taking her to the groomer because I'm paranoid she could contract something there. (That being said, I need to get better with my yorkie grooming skills - Tink's got some "Grinch Feet" going on
)
I guess I will never know what caused this, and like my vet said, it's just an unfortunate part in life. However, she's a happy little girl today so that's what matters for the time being.
Thank you all.