Hmmm...sorry that I don't have any advice to offer.
My DD did do something similar though. For awhile we still were putting a diaper on her for naptime. So every day like clockwork she would poop at nap time (in the diaper) in order to avoid going on the toilet. Not sure why but she eventually got over it and we didn't have to do anything special. We just made a really big exciting deal about it everytime she did go in the toilet.
Hope you find a solution.
-Blessed wife to my wonderful USAF DH
of 11 years
, homeschooling mama to one sweet DD (8) and 4 awesome "all boy" DS (6) (4) (2.5) (14 months)
and adding to team blue to create our "Fab Five" in August