my dog has sudden spells where he shakes constantly and he does'nt appear to be able to move his back legs. it looks like he's having a spasm of some kind and he constantly licks his lips and swallows like he has a dry mouth but he won't drink. then for several hours afterwards he justs lays there and sleeps like he has no energy where he normally runs about and jumps around. do i need to take him to a vet to get checked out?