stress has a lot to do with it. if you were worried about the next day, could be it. what is your diet like? could be that. also, if you are like me, and work in a non AC gym, i intake some salt water before lifting. i know if i skip it as im dry heaving in 10 minutes of the session from dehydration and such.
but like others say, chalk it up as a learning day. and murder the weights next week.