Thanks for the responses. I will try changing her food and if it doesn't help then a trip to the vet.
I'm wondering if stress isn't part of the issue. I brought 2 strays in this winter but kept them separated from my 2 cats. Xanadu pitched a fit and has since turned on Gizmo. I now have moved the strays out to the mudroom and have to keep Xanadu and Gizmo separated because of their fighting brought on by the strays. Argh!
So it could be that Xanadu is acting mean because of the strays or it could be that she is mean because she doesn't feel good. So complicated!