Sounds like the rest has moved like the othr guys have said. What bow do you have? looking down the limbs is one way another way to check if it has a straight rod off the bow like Diamonds and some others do is nock an arrow and look down on it and if it is not lined up with the string stop rod it is usually off at the rest.
Cam lean can also cause this but usually not quite so dramatic.
don't understand why. It keeps getting worse and worse.
That is the part that concerns me a little. Is it getting progressively worse as you shoot it or just getting worse as you move the sight? If getting worse the more you shoot it there could be a limb issue eak on a split limb or warped on a single limb.