This must be so painful for her to walk on!
I think you need to go back to the vet and ask what's going on. It may be a cyst which gets infected from time to time. A cyst, unless completely removed will keep filling up again (it's like a little capsule) can can be prone to infection.
Anyway -I am unsure what it could be but I think you need to take her back to the vet for some advice.
Are her nails cut TOO short by any chance? In which case, infection could get in via the "Quick".
My dog has a reoccurring boil in the same spot every time that shows up a day or two after she gets her nails trim. She just had an xray and my vet said that he saw a few things that might be irritating that area and so he put her under cut it open cleaned it out find the few couple little items --flushed them out. That was two weeks ago. Had her toe nails trimmed and now the boil is happening again. This has been going on for almost a full year. My pour little dog needs to put this to rest. Thank you in advance for any advice.
cc
It sounds like interdigital pyoderma. They develop cysts (although not in the true sense of the word) between the toes and these become infected. The only way you are going to clear it up is with antibiotics, but what type of antibiotics depends on a couple of different things. A culture should be done to determine what type of bacteria is causing it, and then depending upon how deep into the skin the infection is, sometimes a topical antibiotic can take care of it, but if it is deep into the skin, oral antibiotics will be necessary.
Try to keep him from licking at it, because this will keep it wet and in the folds of the toes it won't ever dry real well, and this is just going to keep it going. If he won't leave it alone, put an Elizabethan collar on him to prevent him from messing with it until it heals up.
Make an appointment with your vet ASAP so that you can start him on his antibiotics. Wet spring weather is coming, and it's going to turn into a real uphill battle if you don't get a handle on it soon. Remember to always dry his feet well when he comes into the house from being outside if it is wet out. Pay special attention to the areas between the toes.
Please post back often to keep us up to date on his progress! :)
Ghilly