Thanks for the advice, everybody. It doesn't look anything like the mast cell tumors I've seen so far, and I'm really glad to hear about everybody else's experiences with bumps like this. I'm really hoping it's some type of cyst or some other innocuous thing. We're watching it for a few days and if there's no improvement she's going to the vet.

I don't know why I was so scared. I've just seen so many greyhounds go down with osteosarcoma, so cancer just scares the cr@p out of me.