Toilets on planes are much different than toilets at home. It seems like you could be sucked down a plane toilet when it flushes. Did she go to the toilet on the plane? Are you sure she knows she can't possibly be flushed down the toilet? It sounds strange, but she wouldn't be the first kid to get this idea.
This may not be the problem, but it's probably worth checking out.
Could there be something that she is eating that is causing this? My son had problems one time when he was a little guy from eating a lot of watermelon. However, this only happened a couple of times. I think perhaps you should have her checked out by a doctor again, or look to see what other changes have been going on her life that might cause her to regress. Three is pretty little; she might be upset over some changes in the family or something that is going on at daycare or?
I would get another doctor's opinion, and fast. Either there's a psychological problem that needs to be addressed, or if, as you mention, she's actually unable to control her urination, both could be signs of a larger problem.
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