Has the nail curled around and grown into the foot or is it the width at the base of the nail growing wider?
A groomer "may" be able to help you. If you have a PetsMart near you, they usually have groomers that can maybe take a look at it. If it is too bad or infected, you would probably need a Vet. Make sure all nails are kept cut short.
Try soaking the foot in some warm Epsom salts and water a couple of times a day.
If it is from the nail being too long and growing into the foot, it needs to.be cut and removed. But you will beed antibiotics depending on how deep/how long the nails been in the foot.
That's why I asked suspecting it is a dew claw.
Neighbor of mine came down once asking me to check her Shih Tzu as it was limping on a back leg. That is what I found, the dew claw growing back into the pad. She didn't even know the dog had nails there. Got my whelming stuff out and was able to get under it and cut the thing. Told her to soak it. Yes, she had antibiotics at home.
Ready for this? Her husband was a doctor and refused to let her take the dog to the Vet!!