<human>: Can you condense the text into a few key points?

My daughter Laurie is 16. Everything with her has been normal, except for one thing. Often when I come into her room, just to talk, or to call her down for dinner, or to drop off laundry, etc, she is just sitting in her room doing nothing. 

And I don't mean "nothing" in the sense of watching TV, or playing on her phone, or listening to music. She is either sitting in her chair, staring into space; laying/sitting in bed, staring into space; or sometimes she is sitting on the floor, staring into space. 

As soon as she notices me, however, everything is switched back to normal. She is her usual upbeat self. She holds conversations, talks about her friends, her school, etc. I'll ask her how she's feeling and if she's doing okay, and she says she's doing great. I'll try to gently poke at the subject, by saying something like "You recently have been looking like something is on your mind. Is there anything you want to talk about?" And she gives a very genuine seeming "nope". She acts completely normal around other people, but it's as if I keep catching glimpses of the private version of herself, and I'm getting worried.
<bot>: I keep catching my daughter alone in her room doing nothing, just staring off into space. This is unusual for her, but she is acting completely normal when she is around others.