Without scaring her I would explain to her how serious holding her poopy is. She isn't potty trained until all the steps are done and tell her that if she wants to truly be a big girl she has to go on the potty for poo and pee.
Get her a fun seat cover for the big toilet too, makes it easier to poop on, less mess and makes if fun for them, if you don't have one already.
If you have to take her to the Dr and have him or her explain how sick she can get from holding her poopy in.
She is having power struggle issues more then anything. Keep the pullups on to save your sanity but start with the reward chart again. Some hate pullups but to be honest until a child is 100% ready, it saves you laundry and messes, still enables them to pull them up and down and just worked for me with both of my kids like a charm!
This was the time of year my daughter regressed several years ago, our city center has seniors that play Santa and you can have them call your house. One of them called my daughter and told her he had a lot of toys for her but they were big girl toys and he would be really proud if she went to the potty, well, it was done. She hung up the phone, got underwear on and there was never an accident again. Promise.
Maybe it doesn't have to be Santa but a favorite character.
She is just testing you and claiming independence, however it is dangerous if she is holding it.
I would be matter of fact, tell her you guess she isn't ready to wear big girl underwear and tell her to let you know when she is. Then let it go. She will come around, especially if she has incentive like big girl things! :)