I would say that is tendon damage, had my bitch out for a mooch day time about 6 weeks ago, started limping for no apparent reason, but noticed her to middle toes were swollen took her to the greyhound vet, who confirmed it was tendon damage 4 - 5 weeks on the lead, the best part about it she was walking normal the next day has nothing had happened.
Her toes are still swollen now, and probably stay like that for the rest of her life, I have had her on the beach for 1 1/2 hours twice a day for the last week with no sign of limping fingers crossed