J.O.
You already explained why she is doing it :). As babies learn new milestones thier sleep can be effected, If she is stuck letting her cry isn't going to help...she's still stuck, I would go in soothe her, tell her it's night night time and go out, if she is still upset a few minutes later, go in and repeat. She'll get back into her groove, you just have to understand that the best night sleeper will still have rough periods.