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/