It's a good idea to have some of the purple antiseptic spray for poultry in your chicken first aid kit - one make is called Septiclense. It can be used for wounds anywhere on a bird, and is useful for chickens that have been somehow injured, maybe as a result of bullying, or attack by a predator. It not only helps to clean the wound but the purple colour camouflages any bleeding - chickens will immediately attack a bird with any sign of blood on her. You could use it on the foot if trying what the article calls 'conservative treatment' - wash and dry the foot carefully first, then spray and cover the foot and separate the hen, as directed.
Or, if you do find a big lump in her foot which will need excising, she will need to go to e vet I'm afraid. Don't hang about if you're sure what is wrong, as it won't get better on its own if there is noticeable infection there. Experienced keepers sometimes cut it out themselves, but I wouldn't advise this if you've never seen it done successfully.