Play a lot of video games, by any chance? You don't mention if you're a chronic anxiety sufferer or not, but even by age 15 I bet you've been through a lot of interesting thought processes. Our minds are very creative and active, and in your life you're going to get all kinds of thoughts. If you're a creative person, you'll get even more. If this feels different, have you spoken to your folks about it? I can tell you, in my life, things have often felt like that, so it's not at all an abnormal thought. If it's going on all the time, then again, you might want to talk to your folks about it. Why does it frighten you so much?
My son has anxiety and a bit of depression which is newer. He's 14. He describes what you are talking about as on bad days he goes to school and feels like he is watching himself in a movie. He's separate and observing. Kind of what you describe. He also says on days like that, he feels like he doesn't have control and can't come out of 'the box' he feels like he's in. So, we are actively trying to help with the anxiety. Your mom will want to know what is going on with you I know is hard at your age to think you can share your inner most feelings with her. I think my son felt that too. It took him a long time to really tell me what was going on. But once he did, I think he was relieved. It wasn't easy for me to hear but important that I know. Now we are on the same page working on things to improve this. And guess what? It IS improving.
Now, at your age too, there are good and bad days in general and it isn't a mental health disorder. So, my son's story doesn't imply that your situation is automatically a trip to a psychologist! But you want to work on any issues you are having with emotions and moods early on if they make your life really hard. I remember feeling melancholy and depressed a bit in high school. I have never suffered major depression. I just had a lot of hormones and immature coping skills.
Anyway, that's a really long way of saying, find a trusted adult to talk to. Your parents can help you the most, but a teacher or school counselor can help too. Let us know how you are doing.
It sounds like the dissociation (look up dissociation) I experienced for over a month when discontinuing Lexapro (an anti-depressant). Have you discontinued any medications recently? Has anything major changed? I would be wrapping Christmas presents and then almost wonder how it got done, because I didn't feel like I was really in my body. It was all a fog. It causes even more anxiety, depression, etc. It is a terrible feeling. Something that people take for that, is L-Tyrosine. It's an over the counter supplement. I took it during the weaning process and I do think it helped. If it continues, there's no harm in going to your pediatrician and explaining how you feel! Is that feeling your only symptom?
This is called depersonalization or DPDR. This is common with anxiety and stress response hyper stimulation. It's your brains way of taking a time out from all the rumination and stress.