Hm... well, most basements... although seeming dry in nature, are in fact high in humidity. Humidity being woods worst enemy (next to termites) the wood of the guitar may have expanded in a certain area. One thing that people miss when they diagnose problems like this is the fact that the body of the guitar is made of wood as well. Most people assume that since the neck isn't straight... the problem lies within the neck... Your problem doesn't sound to me to be the neck becuase you say that it's only the bottom that you see the difference. It very well could be the neck pocket of the body. Neck pockets aren't lacquered like the rest of the body... this allows a tight joint between the body and the neck and allows the woods to transfer vibrations... This however can cause problems with swelling. When the pocket becomes saturated it can cause bumps in the wood or small swelling areas that may push the neck from side to side... or even tilt the neck back and forth. Obviously... this is most prevelent in guitars that come stock with less than tight fitting neck joints. The problem COULD be the neck simply aging... wood will expand and shrink no matter how constant the humidity. over a four year period this may have just budged the neck enough to make you notice it. It could be such a thing as if you take it to a tech he may just take the neck off and reset it. But that's my thoughts... it's probably nothing too serious... but most likely a truss rod adjustement wont do anything. Truss rods are made for concaving and convexing and counteracting string tension... not by twisting or moving the neck laterally. Good luck