A.S.
My initial gut reaction is that she's got some sinus drainage making her belly icky. My daughter throws up just from sinus drainage (done this since about age 2)... Especially during her seasonal allergy time. It's not consistant, it seems to be completely unexplained, and during the day she was normal as could be. Also, this could be the beginning of a sinus infection... That would explain a slight fever along with the irratic vomiting.
Oh... And if she doesn't seem stuffy at all... Doesn't necessarily mean that she isn't getting some drainage while laying down.
If it's something like this.... Nada to worry about.
Hope that helps a little. : )