Limping (or lameness), hopping, skipping, hobbling, peg-legged, stiffness in the front or back legs, dragging leg(s) or any kind of abnormal gait exhibited by your dog are all signs that he or she is uncomfortable, and likely in pain.
If the any of your dog’s leg are non-weight bearing (i.e. they won’t put that leg on the ground when walking, or will try to skip/hop on that leg), it’s a good idea to further observe and inspect your dog to determine if it’s an emergency or something that can wait a few hours (or days) until visiting the vet.
Take a video or two of your dog limping, especially if it is intermittent and only occurs after strenuous exercise or running. This will not only help you observe the limp more carefully, but it can help your vet diagnose the situation, as intermittent limping (on and off) isn’t always observed in the veterinarian’s examination room.