My J,r, Terr. At Rest Suddenly Became Nervous And Began

My j,r, terr. at rest suddenly became nervous and began to shake like he was cold but its not. He then began to pant but he was lying down in my chair. The house was quiet. Last night he did the shaking but no panting. He also kept asking to go outside but it was to cold to let him stay out. His appetite is fine and he is drinking well to. This only happens early evening and only the last two nights. Thank You