K.R.
I think I might know because we went through the same thing....you might want to stop using the words "big girl". She very well might not WANT to be a big girl. My son even to this day dreads his birthday because he likes the age he is and doesn't want to change. With her birthday coming up, she might be feeling a little resistant to becoming a big girl. She sees us Mommy Big Girls with issues, stress, busy lives, lots to do and think "Stop this train!!" Then she sees the baby and thinks now THAT'S the life!
Since 3 year olds have no ability to rationalize, all of these things bounce around in her head and manifest as regression.