I agree with miss lydia, it looks like a bad bumble- I would follow her advice on treatment. Please do keep us updated on her!
One of my girls is currently developing the start of bumble foot- I'd recommend checking the living arrangement, make sure the ground she's kept on isn't too hard. You don't want to treat this just for it to come back or affect another one.
My mistake was temporarily keeping mine on very compacted soil- Anything hard, or rough, like stones, concrete, things they can scratch their feet on, etc, can cause this.
Good luck with treatment!