This happened on my 180 HP RV 4 shortly after I finished it. Before having it painted, I built new elevators and the rudder with Van's .020 thick skins and reduced the spacing of the stiffeners. The airplane now has about 550 hours on it and no problems.
Do a search for; Rudder Cracks, and you will come up with quite a bit of chatter about it. There are some posts with pictures too.
This thread shows pretty well what to look for around the rivet heads. Most of the cracks I have seen are far more obvious than this.