For a month or so after a declaw a cat can show signs like this. IT can occur for a few reasons including infection, scab that is causing irritation, surgical glue that is causing irritation, or slow healing. I would inspect the toes on this foot and see if you see anything unusual. In the end, it may be worth a recheck with your vet just to make sure there is not an infection and to make sure there is not some other issue complete unrelated to the declaw. I hope this helps and gives you direction.
It may be worth having an xray done or getting a second opinion to determine if there is a shread of bone left behind. I hope this helps.