As far as I know, planet x hasn´t been discovered yet... There have just been some minor differences between the calculated flight path and the one which actually happened, for two mission. This is a hint, that there could be another big rock somewhere in our solar system whose gravity caused it. Depending on its size (by influencing the mass of course), it has to be in any case quite far away There are a few theories about it, which could be possible. But still only theories, as far as I know.
If wikipedia is right, poleshifting does happen, but it is a VERY slow process... A nearby passing planet or a metheor crashing on earth would probably cause a rapid change... But I guess that is kinda unlikely.