a burr under (one's) saddle

A source or cause of intense and persistent annoyance or irritation. The traffic delays resulting from construction of the new inter-city tramline has been a burr under residents' saddles for months now. I'm not trying to be a burr under your saddle—I'm just ensuring we are following the letter of the law every step of the way. A: "Liz sure seems miserable." B: "Yeah, apparently, she experienced a lot of turbulence on her flight, and it was really a burr under her saddle."

a burr under (or in) your saddle

a persistent source of irritation. North American informal