If there is any swelling it could be a minor infection. If there is swelling/infection I'd squeeze the area to see if it pops then clean well and sterilize. If it is just discoloration, don't worry about it, but do check it occasionally.
It definitely looks like an infection.
It's a bit red/irritated looking around it.
I'm not sure if that's visible in the photo.
Will it cause any problems, do you think? Or will it just clear up?
I could dip her feet in an epsom salts bath.
Thanks for the reply.
Yea, I would give her a bath as well. Don't try too hard to pop it, if it doesn't give then leave it. You don't want it to get more irritated or infected. Keep a close eye on it, you don't want it to turn into bumble foot! But for now, just do what you can to keep her clean and comfortable!