How did you mark your BDC? Notch the fly-wheel and put a mark on the bottom of the chasis to line it up?

If that's the case, then I would say YES, your fly-wheel has slipped and the nut wasn't screwed on tight enough.

There is no "physical" way for the piston to switch BDC or TDC unless you have had a drastic failure with the internal engine parts.

Good luck and let us know how it goes.