Settings / battery usage. Tap the graph - if the awake graph shows a lot of blue, you have an app (or more than one) holding the phone awake. The settings / battery usage first screen may give a clue what that is.
One other thing to look at is a corrupted Google sync. Settings, tap "Google" under accounts. If you see an item with a red circle rather than a green circle, it is having sync problems. That can drain battery fast.