Jimmy seems to have started trembling a lot recently. I know it's started to get colder but it's not been really cold yet. He seems to do it a lot when he's asleep or when lying down relaxing, his legs will be quivering away. Also sometimes when I'm giving him a scratch. He's always done it a bit when sitting waiting for a treat or me to get his lead, he'll sit there waiting with one front paw raised shaking away. Whereas that seems to be down to excited anticipation I can't think of an explanation for why he's doing it when relaxed or asleep. Is this normal ?