She may be having night terrors. Does she stop when you come in or does it keep going on? My daughter started this before she turned 1 and still does it (she's 4 1/2 now). It was really frustrating the first few times before I knew what was going on. She seemed like she was awake but was not. She would be screaming to be up, when I picked her up, she wanted down it would go on and on.
I was told that it's best to not wake them from this episode but in my experience, if I don't wake her up, it continues through out the night. If you think this is it, call your dr. You can get info on line too.
I find that it happens when she is over tired, over stimulated during the day, or both. My niece also gets them (8 yrs old). she will tend to get them when she has had a rough bedtime routine.
Good luck