If you saw a black scab, I'd be willing to bet that it's bumblefoot.
She may not have limped up to this point if the kernel inside her foot wasn't big enough to bother her.... But it looks like it's bothering her now. Bumblefoot is a very serious infection that will require your intervention.
If taking her to a vet is an option, I would start there.... A simple surgical procedure will remove the kernel and a round of anti-biotics will keep any further infection at bay.
If you can't take her to a vet, you can perform the surgery yourself. Just make sure you have gloves!!! Bumblefoot is caused by staph, and staph is NO JOKE. You do NOT want to come into contact with it!
Check out the link posted by beetandsteet... The Chicken Chick has a video of the surgery.
MrsB