When a dog starts limping on there front leg or limping on any of it's legs the limping can be caused by something as simple as a thorn or something else in it's paw.
So check the paw to be sure nothing is in the paw such as a thorn or sticker, needle etc that the dog could have stepped on.
If the dog doesn't have anything in it's paw then the cause of the limping could be caused by things such as a bone fracture, burn, sprain, strain, arthritis, ligament or tendon damage.
Cancer of a dogs bone, joint, muscle, nerve, or any other area of the dogs body could cause the dog to develop a limp on it's front leg or any other legs.
If the limping doesn't stop within a few days to a week then you might want to take the dog to the vet if possible to see what is actually going on.
Sometimes the limping is nothing serious and could be something in it's paw or the dog might have slightly injured it's front leg when running and playing or it could be something else that a vet needs to look at.