C.S.
I've been there too. My now 4 1/2 year old went through this for the same reasons you talk about... new school, new addition to the family.
I think they just need to know that they are still important. All the changes are disconserting to them and they don't know where they are in the world. Know what I mean.
I would spend as much time as I can with her so she gets reassured that you are still there for her-- remember she's still a baby. Hard to understand when you're feeling touched out-- but try to enjoy this time with just you and her. It's hard for everyone with a new sibling in the family too-- so expect regression and anger (more likely toward YOU). With love and patience-- they'll come through it in a few months. Make her feel like a "big girl" on how she can help you when baby comes.
Take some time for yourself too. Have daddy take care of her and leave the house!!
Good luck. It is so fantastic to see your children love eachother!!