Its bumblefoot. This often is from something getting stuck in the skin, and not comeing out. Its prudent early on to soak the foot (set her in a very small cage or the like, in a container with just enough water to cover the injury. Let her stand 15 minutes, then take it out and peroxide. (you have to soften up the tough foot hide). Sometimes this alone will open it enough for infection to leave, and the peroxide can get in and be usefull. I would try this before going on to more drastic steps. It can be so bad as to have to dig something out, but its small, and I'd start small.