My Jack Russell Gets Cramps

My Jack Russell gets cramps, usually after a walk when he is very active. When we return, if I don't cool him down quickly he gets an all body cramp and I have to massage his legs and body until it passes. Yesterday he got the same cramp for no reason except he has some bites on his body and maybe they were stressing him out, but he just started cramping and I have to ease it all out, any clues on what this is.