C.C.
My daughter did the same thing. She was completely potty trained at 2, even in underwear- then I had to go have another baby. She went straight back to the pull-ups.
She started pre-school in her pull-ups. Her teacher and everyone tried to get her to use the bathroom but she wouldn't. We tried charts, rewards, threats, everything.
It wasn't until her big scary aunt gave her a verbal lashing and went off about how she was too big for her diapers did she finally use the potty and go back to her underwear. Instead of a big scary aunt maybe you could have her grandmother or doctor talk to her.