Oh Maya, Banner JUST had the same thing! (twice) ;(
It's on the inside of his left foot, where like our instep would be. It's definitely an abscess of some sort (well, Banner's was) They suspect he may have been allergic to his litter, but we're still waiting to see if that's the case. At first they thought it was just a puncture wound that went bad, but when it came back we've been trying other theories.
It's going to be fine but I would have it looked at sooner rather than alter
My vet lanced it and gave him antibiotics. First time around I cleaned it every day and put neosporin on it but Banner is such a pain in the butt about being held that we didn't do it this time around (yay, no bite marks or scratches this time lol). Banner also hates metacam (go figure), but it definitely was hurting the poor guy.
If you are okay driving to NJ (you moved to NYC, right?) let me know and I'll put you in touch with my wonderful vet. It'd be a hike for you, but if you can't find someone you like/trust, it might be worth it! PM me if you want.