My cousins daughter regressed just like this and it was because they were doing to much discipline with her (alot of spankings) and they had potty trained her to early at age 2 they forced it.
My own daughter was having alot of accidents about a year or so ago and I thought maybe she had a bladder infection or something going on took her to the doctor and they did a x-ray which seemed really weird to me at the time but it showed she was severely blocked up (constipated) and that was pushing on her bladder and the sole reason for her accidents. My daughter got a prescription laxative powder I put in her drinks and to help out with the constipation and now I just make sure she is going regularily and eating alot of fruit. Problem solved.
I would bring her to the pediatrician and just see if anything medical is a reason for this. Can't hurt and it's a little weird if she was fully potty trained and then starts to regress. Another things and this is a touchy subject but I've heard that kids that are sexually abused can have this sort of symptom too and I'm not implying anything just putting that out there.
I wish you luck! My daughter just started kindergarten and at orientation I was told that accidents happen alot in kindergarten and they even have extra clothes at the nurses office for this sort of problem. I know they can't handle a child that is having daily accidents but if an occasional accident happens it's nice to know.