Dog woke up limping

My dog woke up this morning limping on his right leg.He went to bed fine.I checked his paw and felt his leg and he does'nt seem to have any pain.This has never happened before.Does anyone know what could be causing this?I thought he may have slept on it the wrong way but he's been like this all morning.It's his right rear leg.

Right front or right rear leg? If it were my dog, I'd watch him for 24 hrs, and if no improvement or if the limping worsens, I'd make a vet apt.