Kiddo Suddenly Sharting?!

Help! I have a 3.5 yo female. She has been potty trained (even overnight) for over a year. Twice now in less than a week, she has pooped a bit while tooting. What gives? How can I help her to, er, not do this? We talk about not pushing hard when toots are coming, going to the bathroom as soon as something feels ready, etc. Once was overnight, and once was just while at home. Advice please! Cross posted with Parenting Group.