It could be a slight sprain, but it’s more likely he stepped on something. I would check the paw carefully for injuries or something lodged between his toes.
Other than that I would watch him for a few days and try to keep him resting and if he is still painful on Monday I would take him to your local vet.
My suggestion is to visit your vet, it sounds like your dog is starting to fight off some disease and only your vet can diagnose what is wrong.
Good Luck
Dr Dawn