My firend's DD did exactly the same thing. Having lived in Asia and used "squat toilets" quite a bit, my personal theory is that pooing in a sitting position is really un-natural for our bodies. At 3 1/2 my friend's DD started doing poos outside in the garden (when she was getting dressed after swimming) and from there soon started doing them in the toilet. I am not sure whether this was because she started doing them in the garden or whether she just got to the age where she could do it. Either way, I'm sure your DD will get there when she's ready. GL.