Since you're unable to say more, then w/o seeing it for myself, I can only figure that the floor is NOT tied into the foundation and wall systems of the addition. Hence, house is level but floor isn't. You might as well seek the advice of a professional and get an estimate. At least you'll get an explanation as to why the floor is unlevel. I don't know, is it possible that the floor is level, and the walls and foundation aren't, instead??! If you lay a ball on the floor, it does run away from in the direction of the downhill?
If you have more, do post up. I don't have any more info for ya. My best to ya and hope this helps.