Hi C.,
Although we're not there yet, I wonder if the sudden change from being around other kids her age has affected her a bit. I notice my 2 year old really does well with the "peer pressure" of wanting to imitate what friends do. She might be reacting to this change.
You might also consider how independent you're allowing her to be. For example, did she pick up her own toys at school, and are you requiring the same of her at home? Sometimes kids take the easy way out. For her, maybe she feels that she has more attention than she had at school and maybe she wants to see if she can get you to baby her more now that she's not one among many. She's probably just testing the waters in the new situation to see where her boundaries are with you. From everything I've read this is a perfectly normal and healthy thing for a child to do (test boundaries).
If you aren't sure what her previous routine was like, maybe you should talk to the teachers at her school and find out what she was able to do there. My m-i-l was a teacher for years and said that kids are more capable of being independent than most parents give them credit for - perhaps we're reluctant to let them grow up? :-)
I would keep up the positive reinforcement and encourage her to keep using the potty. Be perfectly neutral and matter of fact about her "accidents" but praise her for going to the potty. I probably wouldn't go back to pull-ups during the day unless this is the case. I'd also enlist her help in cleaning up her messes (of course, matter-of-factly, not to shame her) to show her why she needs to use the potty.
Maybe getting excited about putting a sticker on a chart every time she successfully used the potty each day will also help reinforce the behavior you'd like to see more of.
I also wouldn't rule out a medical reason - if it continues you might want to have a conversation with her pediatrician. She might have a urinary tract infection or something.
Wow ... we have so many miles to go yet. Best of luck in getting her back on track!
Smiles,
L.