"mommy, don't leave"

I have been hearing a lot of this lately. My little girl who has always been outgoing and never afraid of running off to play is now turning back to look for me. She is having a hard time letting herself sleep at nigh or go down for naps b/c she is afraid I'll leave her. So bedtime now goes something like this... We have her bath, go to her room to read, rock and sing, say prayers... she gets into bed, asks about the tv to which I say that TV is already asleep. (she will stay up as long as it is on) I rub her back and then she says mommy don't go, don't leave me... I tell her I will never leave her, and will only be in my room at night. SHe tells me again not to leave, I "lay low" on her floor holding her hand. WHen she lets go I stay on her floor waiting for her to drift off. I never fails that 2-3 times she sits straight up yelling " Mommy... mommy, where are you?" If i don't answer she begins to cry. I'm not used to this. It breacks my heart. Of course it makes me doubt preschool. It's great there, but I hate to take her when she tells me not to leave her all the way there. They are great about letting us call to check on her though. And I HATE her being there on fridays whie i am home, but it seems that 2 days a week will help her to adjust better than one. It'd be nice if tuesdays open up b/c then she could go then and stay home on fridays. Wish us luck as we try to get her sorted out. She's going through a lot, and as smart as she is I know she is onto the whole thing.