Rex is a Very Handsome Dog!!

What does Rex usually lie on while he's sleeping?
HARD floor or a SOFT rug or bed?

You might try to encourage him to sleep on something SOFT
for a while - then observe him when he gets up.

You should discuss this with his Vet -
it may be something that just doesn't present to the Vet during
a routine exam -
but if the Vet is *looking* for the cause of the "limping symptom" -
he or she might do the examination differently.

We hope it's nothing serious!