When i stretch i get dizzy? Help please?

It mostly happens when im sitting in a chair at school and stretch the way you would to crack your back. I get dizzy and feel tight (if that made sense). I feel like my body is about to collaspe and my neck feels tight. I drink LOTS of water so idk what it is. Sometimes it feels like my body stiffens up and feels like i'm about to split into two or crack in half. What is it?, Thanks:)