Hi,
My son has always been in a sleep sack at night, just because it gets pretty cold, and even though we have the heat on, it's different when he is alone in bed. Make sure you buy a sleep sack appropriate for her size and she will be fine--especially if she knows how to turn over. I would check her gums and make sure if she is teething or not, babies also go through growth spurts and some researchers say that every time your child reaches a new milestone, her sleep might get disturbed. Her world is opening in front of her and she wants to explore. And since she cannot process emotions yet, she might not be able to relax well at night. Another thing I have read about is sleep regression--it's a phase, that's all you need to remember. How often does she wake up and what do you do, does she want to be nursed/ bottle fed? I still to this day nurse my 20 month old if he wakes up at night(rare occasion), for comfort. Good luck!