M.P.
This happened to my brother too. My mom made up a whole story about "Ocky-Land" - a magical island where all the "ockies" (poops, and pees) go. It is an island so they have to swim there.
Does she pee on the toilet? How does that go?
Silliness aside - I have heard a LOT about kids going through this. Sometimes it is just that the new sensation of controlling poops and being aware of them coming out, feeling them come out is, for whatever reason, uncomfortable for kids. Is she constipated at all? That might make it actually PAINFUL if the poops are hard and HURT her butt when they come out? Or, if she is just scared, I think it is perfectly natural for her to be afraid.
I would try letting her sit on the potty. If she cries and holds in, let her poop in the diaper, but instead of throwing the diaper away, dump the poops into the toilet. I would REALLY praise her for sitting on the potty. Maybe offer to hold her hand while she is letting the poops go into the potty.
Another idea...and some might think it is gross. But do you have any older children? Could she watch them go poop on the potty? Show her how brave they are? Or if not, have her come into the bathroom with you? Show her you are not scared - thats what we do!
GOOD LUCK!!!!