Remember that potty "issiues" are one of the few things that kids have total control over in their lives. She may be feeling out of control and this is the only way for her to deal with it. I'd lay off for a couple of weeks. Let her wear pull ups but occasionally encourage her to pop in the potty. Praise her when she does, but don't make a big deal if she doesn't. Just change her without comment and go on.
If she doesn't want to wear pull ups, make it clear that every time she poops in her panties, they will be thrown away. With both of my boys anyway, this was very effective. And it saved my sanity, because washing the poop out of little underwear is one of the grossest things ever! And seeing her favorite Cinderella (or whatever) panties going into the bin may be just the "encouragement" she needs!