When it turns to a power struggle, time to just give up for now, not forever but for now. Does she have a little potty she could go on? Do you have a fun potty seat for the big toilet so she feels more secure? I say look at her and say "well I understand you don't want to do this. I will put pullups back on you. I don't want you to get sick from not going poopy and you need to let me know when you are ready to go back into underwear" -"when you are ready to be in BIG GIRL pants again, you let me know"..then take a a few big girl priviledges away from her when the time arises. Be matter of fact, end of discussion and put pullups back on her. Let her be and let her tell you when she is ready. If she went before and then started this I would say it is about power and independence more then anything. If she has a fear over the potty then you need to get her on a kid potty, potty seat for the big toilet or let it go for now. The harder you push the harder it will be and it shouldn't be a negative experience.
My daughter regressed and I just let it go until she came back around. She is also an age explaining how bad it is to hold poopy in on her body.
You can try incentives, like charts working towards a reward she wants, have her favorite character call her (family friend or family member). Santa called my daughter shortly after her third bday and it was done, she got her underwear on and we never went backwards.
You just need to see if it is truly fear, fear of what and work that direction to take it or let it go. You cannot force her and her holding her poopy can cause medical problems.
Hang in there, I doubt it will last long if you let it go!
Hugs