Why does it happen?

So, by Monday I was feeling so much better - still hadn't eaten, but certainly more with it than the day before!
I was booked in for a 121 session of training with Di Talbot in the afternoon. Off I went, feeling supremely confident that Bryn and I were invincible. Well Bryn was - but I was rubbish!! I couldn't manage the simplest runs or turns! Eventually, time was up - and on the last attempt I got it right, but as Di said 'Well, that was the best of a bad job really, wasn't it?'
Why does it happen? It's always when someone is watching me critically (and I mean that in the best sense of the word - and having been invited to do so) Well, I'll just have to book another session with her and show her what Bryn and I can do when we are not both hot, tired and hungry....