Thanks for the suggestions, I know it's hard to get an idea of what the problem is with no picture.
I wish I'd had a camera with me, but getting to the area where I generally find them involves walking half a mile up a creek, so it's a risk for me to bring my camera down.

I'm thinking it's probably an injury, but is definitely infected. This one has very pretty red coloring and is always very calm around me, so I'd hate to see her die. Next time I'm down there if she's still around I'll try and catch her to get some pics and see if there's anything I can do.