D.B.
She wasn't fully ready to be diaper-free. You got lucky for a few months, then she went into a very typical phase of either being afraid of the potty or of wanting to hold on to her poop. Most kids just aren't fully trained until past the age of 3. She's in a normal developmental phase, and I understand that you're frustrated because you thought you were done with diapers, but that's often how it goes.
The novelty of the potty has worn off and/or she's really not getting the "full colon" signal from colon to brain. She might have been reliably trained because the potty was fun and she was there a lot, and therefore successful. Now she's distracted by a normal 2.5 year old's life and activities, and she's just not cognitively aware of her body's needs.
You can't yell at her. Don't shame her. Don't show your frustration. It doesn't work. It can make it worse - if she doesn't want to use the potty, she'll hold in her poop and you'll have a big constipation problem, which will cause more pain and discomfort on the potty. Then it becomes almost traumatic to go.
She's in a new phase. She's not being defiant. Take a deep breath, and let it go (no pun intended). She will not go to kindergarten in diapers. She will let you know when she's ready. Don't ask. You will see it in her behavior.
Choose the battles you really need to fight. This isn't one of them, as wonderful as it would be if it were her time to be trained. My child was nearly 4 for daytime potty use, and was wet at night until age 11 - also a normal developmental stage.