the question is do you feel that you get tired after any other strenuous excersize or work.
could be diet, maybe low blood sugar, but without being pessemistic, if it continues, and you are physically fit it may be time for a visit to the GP.
there are several minor ailments that can cause any excersize to take it out of you, even the beginnings of a cold can do that and several other viral infections that are only very mild and short lived, and then there are the more serious but controllable ones like diabetese.
not trying to be horrible but its always best to get yourself checked when something unusual happens, there are so many things that are easy to get sorted if you get them seen to early.