My 3-year-old daughter has been potty trained for almost a year. Lately, she has started to hold the urge to pee for as long as possible (so as not to get distracted from whatever she's doing), to the point that she'll wet her underwear to postpone going. Eventually she gets the urge to go again, but she has already wet her underwear. I've tried telling her to go the minute she feels the urge, but she keeps doing this. Obviously there's been some accidents as well.
It is not unusual for a child to start having accidents after being successfully potty trained because they are “too busy” to go. The other reason may be that the positive reinforcement for toileting success has also disappeared. You may need to go back to stopping all activity and taking her to the restroom every two hours until she gets back into the habit of going when she needs to without a reminder. Another method would be to go back to celebrating “dry days” with a sticker chart for each successful day and a reward for 7 dry days for a few weeks. Then spread the reward to every 14 days for a few cycles then monthly and by then she will likely be back in the habit. If the accidents seem to be happening more frequently a trip to the doctor’s office to be sure there is not a urinary tract infection may also be warranted.