I think she was in the habit of using a potty because it was interesting for a while, and not because her body is mature enough to really know when she has to go. Potty training before age 3 is unusual. You will hear some people say they did it at 1 year or 1.5 years, but it's not really common. The developmental stage when the "full bladder" signal gets to the brain comes much later.
She MAY be stubborn, she MAY be exerting control over you, she MAY be bored with what was a novelty, who knows? The bottom line is, it's not in your control. Put her back in diapers or pull-ups, don't give her stickers, and stop stressing yourself out. Use that time to ignore the whole issue and do something fun and constructive like arts & crafts, nature walks, museum trips, whatever will stimulate her and occupy both of you. She will get back to it sooner or later, and she will NOT go to kindergarten in diapers! Put the potty back in the bathroom where it belongs, and just move on to other things.
My son was past 4 when he was reliably potty trained, and my pediatrician was very helpful in getting me to see that they eat, sleep and potty train on their own schedule, and you cannot FORCE any of them. Totally helped me relax and move on to other constructive activities!