are you sure it's dead level? from what's happening, sounds like it's nose down. you should be able to lay a straight-edge from back to nose on the Fizik saddle, then put your levelling device on top of that. also, which model? if it's the Vitesse or the Aliante, when you sit on it, you should feel something like a cradling, or hammock effect, it is an anatomical design. you should feel like the sit-bones are supported, not pitched forward or back.

are you tightening the bolts that hold the saddle in position enough? maybe they loosen while you ride causing your position to change.

and what about your riding position? are you bent forward a lot? maybe you are stretched out too much & end up over-compensating during riding by scooching forward on your butt.

ask DSO for some help here. pain is not ok, & while discomfort from long rides might be, it shouldn't be happening because you can't find someplace to sit!