I have Aspergers, but I have experienced lower functioning days, too. It's like I'm stuck in slow motion, and every little task becomes a big deal. I have trouble getting tasks started, and even once started, they take much longer to accomplish than usual. I am clumsier, too, I think because I am less "aware" on those days. I tend to accidentaly break or drop things, bump into stuff, and make dumb mistakes, like locking myself out of the house, or putting my clothes on inside out.
Sometimes I have a few days in a row like this, where I have to really strain to keep focusing on paying attention to the meaning of conversations, so that I don't "tune out" and hear just noise. Even focusing my own thoughts becomes difficult at times, because my mind drifts. The thing that helps most is a whole lot of caffeine, though I'll probably have trouble sleeping later on.
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Sometimes life seems like a dream, especially when I look down and see that I forgot to put on my pants.