That you are now not sure about a decision you made and are considering changing your mind. Example: On a gameshow you have the choice between two mystery prizes. You make a selection, then host then asks you at the last moment if you want to change your mind. You start second guessing your original decision.

It means you're questioning your choice or the choices of others. It can happen before but usually happens after you make your choice. Ex: "Should I have helped them? I'm second-guessing myself now."