I did actually with a stray that adopted us many years ago, he wouldnt go outside, and seemed off colour, although still eating, and a few days after I noticed a strange smell and he had an abscess, which I believe was from a bite that had got infected. He had antibiotics and was fine, and back to being the naighbourhood 'tiger' once again.
I definitely think she needs to go to the vets, I think it takes a lot for a cat to show it is off colour as they are so resilient.
Good luck