it could be a thorn or a small cut.

My beagles will get sore feet after a day or two of running if I don' t treat them befor the season opens.

I take a coffee can,put a can of salt into it and a quart of white viniger ,mix well.some of the salt will not disolve.each day I will take each dog and stick both front feet into the can for a min,then the back the same.I do each dog each day for about two weeks befor the season.then every other day or so durning the season.just add more salt and viniger as needed.
This will toughen there feet.if the have a small cut or sore,when you turn them alose,they will roll and slide for a min.(it kinda burns a little).

I learned this trick when I worked at a paper mill,to toughen the hands so the paper wan' t cut as bad.you will cut up some also if you have a cut.it will heal and toughen the hands(and feet)...........this is not a heal all if the feet are cut bad,or has a thoen in it.........by all means check them closley and go to the vet if needed.