I don't have much work experience, but I'd have thought if you're getting such good reviews then your boss is unlikely to think you're not being productive. I'm volunteering as an administrative assistant at the moment, and there have been times when I've had nothing to do - it stressed me out, too, for the same reason you said...plus I was worried that maybe people just expect me to know what to do instead of waiting to be told, but most of the time I didn't! My manager has been happy with how I've been working, though, and hasn't ever sounded disappointed that I haven't been working hard enough.

Is there anything you can do on Tuesdays and Wednesdays to keep yourself busy? If I'm doing something, however small or boring the task may be, it stops me worrying that people don't think I'm working hard enough. I often spend my spare time going through the Microsoft Office tutorials online - not very interesting, but I can learn some pretty useful things!

Hope I've helped a little, and welcome to the forum!