Has your computer been updated? I have a Bernina 750 QE that would change stitches when I would get up to press a block. It changed from a straight stitch to a zig zag and no, I did not bump anything (like the tech asked at the store) and I am not crazy.
The machine was behind on several computer updates and the tech cleaned and oiled my machine and updated it, and it is back to sewing wonderfully again.