Need a photo. But sounds like an abcess. She may have cut or punctured her foot, or the flap of webbing between toes if she has that. If it’s an abcess and you want to take care of it yourself, glove up, get s sterile razor and a filth rag, and cut that sucker open. Id do it from the top myself, so she can’t traipse through poop and fill it up with filth. Drain it and flush it out, keep it open and draining for several days until it heals. Keep her clean and keep poop out of the wound. If it’s an abcess of course! Chickens are super tough and heal very fast. After the first chicken I had to stitch up I learned that they are far tougher creatures than I! She’ll be fine with the operation if that’s what it comes to.