Did they inject her with anything for her spay? That part of her back is normally where they do those, so my first thought would be that she's either A: has been having a reaction, or B: they used something that caused trauma to the injection site. Injectable Baytril and a couple of other medications can cause necrosis [deadening] of the tissue at the site if they're not diluted enough. May be worth calling the vet to ask.
Is it still getting bigger, or has it stayed roughly the same size? I would put her on a course of Baytril, if she's not on one already from her spay, and call your vet to ask them about it. Ask if they may have injected her with something, or anything along those lines. Keep an eye on it, and if it gets any larger, get her to the vet immediately.
