This has never happened to me before but maybe you have performance anxiety? You said you had done well in the interviews and you're an experienced programmer.

This has never happened to me before but maybe you have performance anxiety? You said you had done well in the interviews and you're an experienced programmer. By listing these reasons you make appearent that any person with your qualities must be able to produce outstanding work. By looking from that perspective maybe you're imposing expectations that are too high about yourself instead of just letting it go and enjoying your job. So maybe treat yourself as a newbie programmer who's just started his career, and instead of trying to do outstanding work you should try to gain as much "experience" as possible.