Could be any number of things - including lack of sleep. Could be diet related...could be overtraining related...could be purely psychological..you could be coming down with something.
I can usually gauge how my workout is going to turn out based on my warm up - like me last week...i've got the flu, which i didn't realise at the time, and attempted to do my usual workout, things weren't going according to plan and i was exhausted during my warm up before I had even started anything. Went to the doc and he said I had a 20-25% reduction in my lung capacity due to the flu. I've gotten progressively worse as the week went on and I ended up flat on my back in bed the past few days with nothing better to do than make useless comments in the forum