Have their been any changes in her routine or surroundings? Our son has gone through (and is going through) the same thing. He potty-trained early, esp. for a boy, at just over 2 years old. Now at almost 3.5, he has started going in his pants again. Unfortunately, it just happens to be at day care. He hasn't been in "school" for a few months, and in all the excitement of playing with his new friends, he just doesn't want to take time out to go potty.
The first time he went through this, we would just keep an eye out for signs that he had to go just like we did through potty-training, and would tell him that he could either take time out of his playing to go to the potty or he could have time out. I know a lot of people will probably think this is harsh, but it worked well. He just didn't want to stop playing long enough to go potty, so we gave him the choice. We only had to send him to time-out once when he was testing the theory, and then immediately he decided he would rather go to the potty. It worked really well for us.
Unfortunately now he is in day care again, and so we have to go through the "being a big boy" talks..