This could be any of a number of things. Symptoms of arthritis can come and go...or this could be a tick-borne illness...? Or it could simply be that he had a strain in that right leg, and muscles and tendons in the left over-compensated for that weakness, causing some upset there which showed up later. That is quite common.
I would definitely take him to your vet, and have some tests done, and X-rays if indicated.
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