Totally guessing here, but could she be sleep-crying (for want of a better term)? Her actions sound like a sleep walker - I remember my sister coming out into the lounge and *watching* TV when she was little, but she just stared without blinking, muttered "mmm" when asked questions and didn't believe us the next day when we talked about what happened the previous night as she had no recollection of it.
At least she cries, so you can help her with toileting and avoid changing wet sheets
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