is my adopted cat ok?

We adopted a cat from a shelter. She is 8 yrs old. It's been 1 yr now. When she sleeps (2 times now within the yr.) I notice when she sleeps she is not breathing. I freak out and try to shake her. It take a minute or 2 to see her chest moving again. And another minute or 2 to get her responsive again. It's this normal? I read a lot about cats falling into deep sleep but nobody mentions the cat not breathing(or so it looks)

My cats do this and it freaks me out!  Unfortunately, I think it's normal.