I myself had problems with this AT school in first grade when I was 6. It was almost every day for awhile, and my mom would have to come bring a change of clothes and help me with it. For me, I think a lot of it was adjusting to a new school, being an extremely shy and timid child who was afraid to ask her teacher to go to the potty (in K we had a restroom attached to the classroom, so that was my first experience with not having that), plus adjusting from 1/2 day where I could make it w/o going to all day where I could not. They had to be very patient with me, and work with me gently, and eventually I gained more confidence and was able to ask when I needed to go.
I wonder if your dd is sensing the changes coming with the new baby, and this is caused in part by the stress of that? I hope you can find a solution, but wanted to share my experience, and let you know that you CAN work through it (I am now 35, and have not had an accident in years and years.