Looks like maybe they caught a nail in something and tore it out or broke it? My cat decided to sneak into my bedroom last month, found her way into a closet and somehow got herself stuck INSIDE a suitcase. I was lucky to hear her cries, get her out and then deal with all the little bloody cat footprints, because she tore her nail.
If its not actively bleeding, I would say keep an eye on it, make sure its healing, but at first sign of an infection, get her to the vet.
