Older Child Regression (?)

Florence
We have two wonderful children: a 5 year old little girl who just started kindergarten last week, and a 2 1/2 month old little boy who is the sweetest thing ever. Yesterday I noticed that our daughter had peed her pants. When I asked her about it she said she was waiting to use the bathroom at her grandparents house & couldn't hold it. I could see the validity in that so I let it slide. But today she was playing behind the couch then darted up and ran towards the bathroom. When she ran past me I immediately smelled poop. Lo and behold she had totally pooped her pants! Stuff like this hasn't happened since her potty-training days when she was around 2. 
Could this be related to having a new sibling AND starting school? A week after she started school I also returned to work, so daddy has been taking on a lot of the duties at home and I only get to spend maybe 2-3 hours a day with my family, which could also be another adjustment for her. Any info or personal stories you have would be appreciated!