L.T.
I think she is going through this because you are pregnant (and possibly also because you work.) She wants your attention and reassurance that you will always be there for her no matter what. Even though it is hard, try to go to her every time she calls for a while - like a few weeks if you can. After a while she may start to feel more reassured and relax on the calling. At which point Daddy can step in before you are called more often to head her off and get her used to him helping her more. The fact that it happens in the middle of the night means she is really feeling insecure. Try to handle this now before your second child comes when you will be less able to deal with her needs at a moment's notice.
I am also a working mom, with a girl who is 2 1/2 years older than her brother so I can relate and have seen this before! It may happen again in the future, but goes away when she feels reassured. At least that is how it has worked for me.