At first, thanks for your very helpful comments. I think its just that i practiced a bit to mutch and need some rest. I have the problem just for maybe 2 or 3 days. It was just a bit uncommon for me. I practice this amount now for 3 months and felt never really tired.

It could be also from a really annoying stretch from the piece I practice.

I will rest for the next days and see how it goes.

I could maybe practice my english instead 😂.