Then the legs starts to collapse

It would happen several times a week when I was a kid. Even without standing up, just by existing. I became pro at it and would just sit on the floor wherever I was when I felt it coming. My friends just waited there till my brain had blood again, they were chill. I'm an adult now with grown up heart in a grown up body, so it's not fun anymore. I used to think it was fun.

Why does this happen and is it dangerous I’ve always wondered