tender spots on my cat's back

My cat, Reno, who is an indoor cat, has been experiencing tenderness on her back-right over her hind legs. When she is petted there she arches up her back and sometimes meows. It sort of feels swollen there--a couple of inches before her tail. Could this be a kidney infection?

Is she fixed? If not that might explain it. It could also be a normal response to petting/scratching there, the arching is common. However if this isn't her usual behavior then get her in to the vet, it could also be kidney, bladder, tumor, cyst, etc.