I think everyone feels these things but with different intensities.. its normal..
i guess fatigue comes when you are unable to relax and get too much stressed up..
just relax.. know that no single question determines the score.. it is the total score that is important.. and there are actually quite a number of easy questions spread throughout the paper... it may be a good idea to skip complicated questions initially...
don't make it a matter of pride to do certain questions like say QM or classical mechanics...that often happens to me..
all the best