I think that what I am experiencing is called an anti-climax. A low that sets in at the end of a course - the two years of hard work is over and the final show is over. The end of any phase in life can be hard and when something big ends you are left with a hole to fill, a temporary (but real) loss of purpose, and the big question - what comes next?
On the bright side the end of a course brings a great sent of acheivement and a future bright with possibility, where anything can happen. So I should shut up moaning and find my purpose (or should that be porpoise) in life.