Dog suddenly ticklish or...?

My 11 year old lab has suddenly become very ticklish or is it something else? Rubbing him on his belly or even chest causes him to kick one of his legs non-stop as if being tickled. When I rub his chest and he does it, he also stretches out his head as if enjoying it. But someone has suggested it could be related to his nerves as he was never like this before - can a dog really become ticklish overnight or is it something else?