Why does it keep shifting it's orbit?
Let me try and explain this. The earth is rotating on its axis and the space station is revolving around the earth as well. However, the space station revolves around the earth at a very high speed and if I can remember it is something like 14 or 16 revolutions in a day. That means on each revolution the ISS doesn't fly over the same area of the earth meaning you will not always see it.
However, like every other object that revolves around the earth it is subject to the gravitational pull/decaying orbit. So the ISS has to keep on recalibrating to maintain that orbit around the earth.
So, no. The ISS does not actually change orbit otherwise it will come crashing through the earth's atmosphere.
That is the best I could do from what I already know.