most likely Bumblefoot, which can cause a staph infection..
for now..get her on soft clean bedding in a confined area where she is warm and safe..with food and water nearby so she can rest the foot.,
you will need to get an antibiotic..options are:
Pen-G or Baytril..(Baytril comes from a vet..Pen-G and other penicillins are available at most farm/feed stores)
other meds:: lincomycin, amoxicillin, cephalaxin..
giving the systemic antibiotic will give you a head start to getting rid of the infection before further treatment, such as lancing..
the foot will have to be cleaned, possibly lanced..usually there is a plug inside the swelling that should be removed.
Diluted betadine will be needed to clean the area, along with neosporin..
if you are squeamish about doing this..she will need to see a vet..
you can try making a poultice of sugar and betadine, applying it using a small cotton ball, and covering with stretchy bandage.
but what you describe sounds like it may need more treatment.
there are many posts with pics and treatment suggestions here for Bumblefoot..you can do a search.
do try to post a pic...since it's possible it could be another disorder.
others might reply with more suggestions.