C.B.
I have potty trained many many children over the years of teaching preschool & I would absolutely NOT recommend putting her back in diapers. This kind of regression is perfectly normal when a new stress come up. My own son went through the same thing when he started preschool for the first time at 2 & a half after having been potty trained for months. What you need to do is stay positive with her. Don't get upset, just be very matter-of-fact and have her go to into the bathroom with you and help you empty her poop out of her undies & into the toilet. Then go through all of the clean up with her & explain to her the whole time that poop belongs in the potty & she is a big girl now & can use the potty to go poop just like mom & dad. Don't let her see you getting frustrated or angry about it. I recommend two books to help you- For her "Everybody Poops". It's a very funny children's book. I paid attention to what time of day my son was having accidents & then started having him sit on the potty right before that time to try & go poop while I read him the story. It worked very well. I also made up a song to go with it. For you- "Potty Training for Dummies" Everyone laughs when I recommend this book, but it employs the same principles I've been using in preschool for years & it's very easy to follow.
Good luck!
C.