Bad Workout

I had a really bad workout tonight. Not the kind where I wasn't pushing myself or goofing off. It was the kind where I literally left the trainer's side crying. He kept asking me what was wrong with me. Why wasn't I putting forth my best effort? Did I eat enough today? Was I sick? Did I have a bad day? Any/all of the above I guess... I don't know what in the world is wrong with me. I felt so frustrated and defeated leaving there. Sheesh...