On somebody's tail - Idioms by The Free Dictionary
on somebody's tail
on (one's) tail
Following or pursuing very close behind someone. The cops are on our tail—we have to lose them in the woods. Why is this guy sitting on my tail when I'm already going over the speed limit?
on somebody’s ˈtail (informal) (of the police, a spy, etc.) following behind somebody very closely: I had the feeling there was someone on my tail.