Ringworm. It looks like it's gotten huge, unless that picture is really close up, it also looks like it's on the calf right below the knee. Possible sources - any sort of domesticated animal, most commonly a cat. So if she has it, your pet has it most likely. Causes, weak immune system.
See a doctor, get prescribed some ointment (most like prescription here in the states). Three times a day until gone. You are susceptible if your immune system sucks.
I got it. 6 weeks for removal, and that's considered fast. I had one spot. Good luck, and don't scratch.