My three year, who was potty trained, has started soiling her pants for the last two weeks now. What could be the problem?

It's quite normal to have regression in using potty. There are some reasons e.g. “an abrupt change in routine, starting a daycare, welcoming a new baby into the home or any other major life change" https://www.pull-ups.com/potty-training/regression-is-normal/1143 Try not to over-react and give gentle reminder she needs to use potty for pee and poop. After some times she will be back to normal. http://www.parents.com/toddlers-preschoolers/potty-training/problems/potty-training-regression/

This behaviour is known as encopresis. It happens after the child has been toilet trained. Do tell your child that it is nothing embarrassing if she has to go. She just has to alert her teacher or parent so they can get her to the toilet in time. http://www.askdrsears.com/topics/parenting/discipline-behavior/bothersome-behaviors/soiling-pants

It's quite normal, much more common in boys than in girls. We can hypothesize that this is because boys are encouraged to keep their feelings in, so they are less likely to ask to be excused for toileting when away from home. Just a gentle reminder would do the job and fix the habit.

This happens and had happened with my kid as well, there could be some change in routine so just have your kid go through the training steps again. This article explains 3steps to train your kid, it helped me https://sg.theasianparent.com/3-days-potty-training

Just gently remind her what to do, with habit it will be fixed.

thanks