It sounds very much like an abcess, which can pop up really quickly. The kitten really should go to see the vet, as it will more than likely need antibiotics and possibly need the abcess lancing. However, if you leave it the abcess will burst when it is ready - not very pleasant Feral cats will regularly get abcesses which then burst and then heal themselves, so it is possible that this will sort itself out without any medical help. Antibiotics would help clear up the infection, but hopefully it will clear itself although will take longer without medication. Unfortunately sometimes complications can set in if the infection is really severe. If the kitten will let you try to make sure the area is kept clean, especially once it has burst. Good luck, I hope she recovers well.

The other option would be to take the kitten to a rescue as she has no owner. The will treat the abcess and then rehome her.