Thank you for your replies.

I think I had a mixture of almost everything both of you have mentioned. The last days I've been practising more than I use too, I feel I possibly could have a small virus coming on, and I might have been playing too strenuously during long runs. I have also (I realised this today) not really relaxed when not sitting in front of the piano. Like in school, I tend to do things with my fingers all-the-time, either play with the pen, etc., out of pure boredom.

(I've also had a few 5hour tests in school.. a lot of writing..)
But I really relaxed today before my lesson, listened through the piece once, played it once, bought a chocolate-bar, ate it 15mins before my lesson to get my energy going, and when the adrenaline came (along with the nerviousity though) when I was about to play for my teacher, everything went
much better. I do think it wasn't as good as for some days ago, but my teacher said he really saw improvement in many places.
So I guess the conclusion is, I have to relax more all day when not being at the piano, remember to play less strenuously, and take a day off once in a while.
Thank you again:)
Joachim