Are my trials and tribulations caused by sin in my past?
No, for the believer, trials and tribulations are not caused by past sin. The bema seat is not a judgment for sin; it is like an Olympic judges' platform for rewards. If you are saved and a child of God, your sin is not still on your back, so trials are not punishment for it.
Les explains that the Greek word for judgment is bema, a raised platform used in the Olympic games for judges determining first, second, and third places, not a political judgment seat. He stresses that we must all appear before the bema seat not to be judged for our sin. When people ask if their trials and tribulations come from past sin, he first asks, 'Are you a believer? Are you a child of God?' If they say they are saved, he asks where they get the idea they still have sin on their back. The bema is about rewards, not sin punishment.
For we must all appear before the judgment seat of Christ; that every one may receive the things done in his body, according to that he hath done, whether it be good or bad.