A.M.
Your daughter is just developing a normal human sleep cycle. Every 90 minutes or so (varying by a few minutes from person to person), you partially awake and re-situate yourself. Now, imagine in that partial wakening, your pillow has slid off the bed. You reach over, grab it, and fall back to sleep, without really waking completely. You are the substitute for the pillow while your daughter falls asleep. When your daughter wakes, she is looking for her "pillow," and when you aren't there, she cries for you to come back. She needs to fall asleep on her own, and in the place where she will spend the night. It will be easier for her during those "partial wakenings" in the middle of the night if everything is the same as it was when she fell asleep.
I highly recommed the book "Solving Your Child's Sleep Problems" by Dr. Ferber. It was a real sanity saver for us. In a nutshell, Ferber recommends that after you put your child down, she will cry of course, go in after 3 minutes or so, comfort her, and leave again. Then go in again 5 minutes later, then 7 minutes, then 10. But don't go to 13, just keep going in in 10 minute intervals. Eventually, your daughter will fall asleep. (Set a naptime window of, say, 3 hours. If she doesn't sleep, then she's just missed it.) Also, waker her at the usual time in the morning while your "training." It will only take 3 nights, and you'll have a good sleeper. We noticed that our child was a significantly happier little girl once she was putting herself down for naps and bedtime, and sleeping through the night.
Hang in there, mamma!