That is a possibility that the
mast is not plumb, but if it was ok some time back, that doesn't explain the 5% now, unless some shrouds are lose, but you'd certainly notice that.
Really hard to say without actually being there.
You say the mast alignment is out of whack, but how about the waterline. Can you see any difference in your bootstripe from either or port or starboard side, I'd start there, though a mast out of whack could also cause the water line to be out of whack.
Were it me, I'd try to give it a careful eyeball from the front and back of the boat to see.
Here's another thing you can do. Take a
halyard from the top of the mast and move it to either side of the boat and measure it to the top of the rail on both sides. See if there is a difference in length. Just place a tick mark on the
halyard where it touches the rail. If there is a difference, the mast is likely out of plumb. If there is no difference, the problem lies elsewhere.
You may have to do this with two halyards if they lay side by side at the top of the mast and get an average.
Be careful not to pull on the halyard as it could stretch, just lay alongside.