It's passage of time.
Sometimes I look at my younger sisters and I can't recogize them. We have big age gaps betweet us so I always think of them as children. Just yesterday I met my sister and even though I live with her seeing her in front of her/my former high school with her friends I almost didn't recognize her... It hit me that the child I use to babysit is a young woman.
It was one of the moments that you mention when my whole body started feeling hot/heated, and that I was alone in the room and was totally aware of it
. Just like that - almost panick attack.
I also get overwhelmed just seeing the number 2010, it seems to me that I should've been a different person based on how much time has passed
Some of my friends also have kids and weddings and engagements and all those serious adult
stuff and it's not that I haven't moved in any direction since high school but still that just makes me think - where did the time go?