D.F.
Wow ! It sounds exactly the same as what we went thru with our Son ! He slept perfectly but then about two weeks after he as moved into his big boy bed, (he was 26months) it would take me about 2hrs to get him to sleep and then he would scream in the middle of the night 2am to 5am. Geez, my hubby and I almost got divorced ! It was brutal!!
So here's what I did, (it takes work and patience...but it does work) I followed the Super Nanny's advise.
When putting him to bed, I would stay in the room with him for 15 minutes or so, sitting at the foot of his bed. (I would briefly reassure him that "Mommy is here" "Mommy isn't going anywhere". Then stopped talking. Did this for a few days, then I moved to sitting on the floor by the door.....then a few days later I would stand by the door, then a few days later I moved a chair into the hallway outside his room, and reassure him "mommy is here" and then I slowly moved the chair down the hallway. It took about 2 months !! Now he sleeps great again ! :-))
As for during the middle of the night, I really can't remember much - it all seems like such a blur -. but I did try a white noise machine......and I would sit at the foot of his bed. and reasure him...etc.
(all this between screaming fights with the hubby, because all he wanted to do was bring him to our bed - and I refused.)
Check out the Super Nanny ! She is great !
Basically, your child is growing up and the world around her is getting bigger and she needs to feel reassured and comforted. This is what comes with thier independence - they regress a bit and need extra love.
Good luck !