How old is he? Perhaps he's having an arthritis flare-up. My Bub limps when his old bones are giving him trouble.

Maybe he took a step wrong and hurt a joint or something... If it doesn't clear up in a week or so, go to a vet

It could have a broken toe nail, mine does that if it breaks a nail.

There are several different things that could be causing this. He may have picked up a (DAMN) FOXTAIL...if they grow where you walk him, or in the yard...anyplace he has access to..Once in the (DAMN) Foxtail (which is shaped like an arrow) simply moves in one direction...forward..into the foot, up the leg and can eventually reach the heart...which is why I feel compelled to call them..DAMN FOXTAILS...I hate them! It can also be a result from some sort of muscle strain. Depending upon what type of collar and leash you normally walk him with, and if he normally pulls when being walked, he may have pulled a muscle in his neck, shoulder, upper back, or chest. ALL of these could result in a bit of a limp. There could be something, such as a lump, internally, where you can not (yet) see it from the outside..(not horribly likely...but I did have one client, whose Springer had this happen...surgery removal...problem solved). He may have gotten a STONE BRUISE on his pad, no visibly evidence now..but the foot is a bit sore when he normally walks on it. (You would think that pushing on the pad would show a tender reaction by him, but remember most animals DO NOT complain of discomforts to the levels we puny humans normally do!...they tend to be far more stoic about things) I would say keep things low key..no long walks for about 48 hours...if there is no positive change at all...and he appears the same...Take him to the Vet, so you can try and get some answers..no matter what experiences all of us here may have...we cannot see your pooch! I hope it clears up soon! Poor fellow!