I would say it's just coz she's 'waking up'. When they're newborns they're so little and sleepy that they can't help but go to sleep when they're tired. But they when they get a bit bigger they are able to keep themselves awake for longer. She's probably just realised 'hey! I'm not all snuggled up inside my mummy anymore!' and it's strange for her being alone in her cot when she's been part of you for so long. I imagine that would be pretty scary for a baby. Have you got a sling?? Life saver for me. DD2 is napping in it now.