If this is an infection you'll need to treat it asap. She could have had a cut that got infected I don't know but even soaking in warm epsom salt might help bring down the swelling.
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if it is swollen and not an obvious injury, give her 3 days of antibiotics and massage foot with some vicks salve after cleaning with epsom salt water.
How about gout? just read that chickens can get it. Maybe you need to evaluate what she is eating. can she get to fresh water anytime she wants too? What are you feeding them? Including treats.