All the time and it weirds me out. I didn't do it as a kid; I didn't start doing it until I reached college. Makes me wonder if it's a part of, or a remnant of, depression and/or anxiety.
Definitely can be. It's your body trying to control the future situations to not get emotionally hurt like you once did, by trying to map out all possible events.
It just ends up as 10 negative thoughts instead of 1 potentially negative experience, so overall it is not good for you.