C.,
Is there any chance she was deprived of oxygen during your labor? Sometimes that can cause retinal damage which keeps the body from understanding the difference between night and day. If this might be an issue, talk to your pedi. I won't say more about it if your labor/ delivery was uncomplicated.
It is also possible that she is not getting to bed early enough. It is possible that she is hungry.
Kids this age need 10-12 hours of sleep (including the nap), and moms certainly need them to sleep that much as well in order to maintain your sanity!
Is her stomach upset? Is she frightened? You say it isn't fear of the dark. Is her screaming then just a way to make you get out of bed?
I'd say check on her but don't let her out of bed and don't stay with her. She's old enough that you can explain to her that she has to stay there. Stick to your guns on this. If she gets up put her right back down and leave the room. Don't get mad at her, just be firm and consistant.
And I'd switch Pedis. It sounds as if this one isn't listening very well.
Good luck. I hope you all get some sleep soon.