Am I the only one... So tired

Am I the only one that every once in awhile you go to the gym or go to work out and then you step on that elliptical take three and wish you were dead/feel like you are dead.   

I find these times very confusing cause seriously the day before I can hop on a machine and go 2 miles no problem but today i can't even go 3 feet without wanting to curl into a little ball and watch tv.  I seriously don't understand how this is even possible. 

Weird !