You are probably in an overspeed stall. Check to make sure your speed isn't in the 'red zone'. Lift your nose and drop the collective slightly.
I found this problem when automatic trim is disabled, try changing your difficulty to trainee and see if the problem is still there.
thanks guys, ill try to fix it like u said, overspeed stall could be right, because i think i flown a bit to fast and i was in the overspeed.
to the 2 Answer: im already in Trainee mode
So as said ill try the advice of Jedra, and post the result then. Thanks!