Chihuahua teeth chattering?

I have an 11 month old chihuahua he had fleas for a few days i got rid of them gave him flea med. this was about 3 weeks ago he has chewed up his bit an back to the point were there is no hair he will not stop chewing on himself! he met my fiance's cousin the other day and his teeth started to chatter..he was just trying to eat soemthing he wasnt supposed to and i told him no and they started to chatter again, he deffiantly does not get beat or anything he doesnt even get ed there is nothing new in the house or anything no new pets nothing, is this normal or should i be worried? he acts perfectly fine very playful as usual just kind of weird that he does that! thanks!