Limping is a sign of pain and discomfort. The lameness can have various causes and therefore a physical examination is usually needed to make a correct assessment of the case.

A young dog with intermittent lameness, even if it is mild and only seen very occasionally, should also be physically examined within a few days. Keep the dog rested until examined by a vet, i.e. rest the dog on a lead and do not let it play with other dogs or run loose. Any jumping should be strictly avoided also. 

I therefore recommend that you contact your veterinary surgery during normal opening hours.

If your dog become affected in other ways, for instance being off food, shivering, being lethargic or if the dog is unwilling to bear weight on the leg at all, an examination should be carried out urgently - Contact your local vet or another veterinarian for an emergency appointment.  Keep the dog rested as above until examined by the vet.