Looks like it could be the start of bumblefoot - which is usually on the bottom of the foot - or it could be simply something she stepped on causing an abscess. You can get suggestions on really complicated treatments for bumblefoot (which is basically a staph infection), but I'll give you my simplified treatment.
Soak the foot in warm water to soften the sore/scab (epsom salts optional). Once softened use a pair of tweezers to gently peel the scab off and get out the "core". At this point I apply a product called Vetericyn...an absolute must in any pet/livestock first aid kit, in my opinion. If you can't get your hands on any Vetericyn, apply an antibiotic ointment such as neosporin without the lidocaine (that's important!). When using any ointment it's necessary to apply a bandage of some sort. Be sure the bandage is not too tight.
As I said, I use Vetericyn, available at quite a few feedstores and Tractor Supply stores. Expensive, but one bottle lasts a very long time and is great for every type of wound. Vetericyn will kill the staph. You want the gel that comes in a pump bottle, not the spray. Vetericyn gets used in this house for every creature - chickens, ducks, dogs, cats...and yes, my husband and I use it to treat our own wounds. They just came out with a version for humans, just gave it a nicer "humanized" name. With the Vetericyn there is no need to apply a bandage; just hold the bird for a few minutes to give it time to dry and it will form it's own seal. Reapply for 3 to 5 days.
Good luck. Gritsar aka Owner of Sunny, the Bumblefoot Queen.