You need to have this evaluated by a vet, and not wait it out. As others have posted, a parital list of possibilities includes (in order of scariness):
-an insect bite
-a thistle or bur embedded in the paw
-a muscle strain
-a dislocated shoulder
-a full or partial ACL tear
-osteosarcoma (bone cancer)
The last private surrender dog that came into our rescue had a small limp. The owner swore up and down it was due to a thistle in her paw and that his vet had prescribed antibiotics. In reality, it was the first sign of hip dysplasia in a young dog--there was no thistle; we had to have x-rays done to find out what was really going on (and no vet had prescribed any antibiotics--he was treating what he thought it was using his "intuition").
You've got to let a vet check it to know. You can't treat on your own limps that have no obvious source. You need to first know what's going on.
Last edited by Magwart; 07-15-2014 at 10:10 AM.