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

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