The back of the hair is usually the first to fall out because of how much time they spend on it, it will grow back in no time, my baby lost her sides of her hair lol so she had a Mohawk
Do you put those little bows around her hair? If so that could be rubbing out her hair also my baby's hair was doing the same thing and I stop putting those little headbands around it and it started growing back
its from sleep. let her sleep in a soft knit baby hat. ..had the same thing with my first. if u don't catch it now it will be harder to grow back....but it will