Hi Nil,
I agree with Memonge. To me it sounds like she is a little worked up about something. Kids get like this so easily as the big bad world changes from time to time and there is nothing they can do about it. It can be confusing and worrying for little ones. Things that seem insignificant to us and we probably wouldn't notice can seem like a massive thing for kids.
I know you said that Princess did it before you were pregnant but maybe there was some other stress at that time in her life?
At the moment she has a new brother on the way, a new country to move to (and she probably notices Mummy and Daddy being a bit more stressed than usual even if you do try and keep it hidden from her) and she's got to the age where she starts to question everything.
I really wouldn't worry but agree that a trip to the doc won't do any harm and will put your mind at rest.
P.S I hope she is not a member of the Twilight gang otherwise that offer of babysitting if you need a break is void