Hi,
I'm guessing that the X and Y axes are no longer at exactly the same scale.
Imagine a circle of 100mm radius but where the X component suddenly increased its length by 10%. You would
end up with an oval, 200mm in Y but 220mm in X.
Looking at the circles you've produced can you see an oval shape? If so might direct you to a faulty axis.
While your description doesn't sound like the classic fault....it could be and worth checking.
On the Config/General Config page ensure IJ Mode (approx centre of page) is checked 'incremental'.
Craig