2nd guessing can happen after making a decision because you aren't sure it was the right decision, or that you think you made the wrong decision. It can be about 2nd guessing your motives, your values, your beliefs, anything.

I might feel very confident one day at work, then feel like an utter failure the next because I doubt the things that made me feel confident, and I diminished them in my doubting, until they didn't exist anymore.

Mostly though I experience 2nd guessing about decisions.