Have been on 60mg prednisone for the last 11 days now tapering down to 40mg for 5 days, feeling very fatigued and strange. is the drop too quick?

Sounds normal: Such feelings are common on high dose corticosteroids (prednisone etc). It's probably not because of the taper, but the high dose you have been taking. Or perhaps the problem being treated. If in doubt or you remain concerned, contact the doctor treating you.

Prednisone : Prednisone itself can cause body ache , tiredness , disturbed sleep and reflux . Short course of prednisone can be tapered the way you are doing .