Yeah I experience these feelings. And like you said, its weird b/c youd be fine the day before, and just wake up completely different. But you definitely have your good days and bad days.
Even though these "not there" feelings seem random, its actually your bodies way of reacting to anxiety. I learned this in a psych. class, and like a post above mine said, its called disassociation. After your body's continously running on overdrive for hours on end (your nervous system is working double time, and this is why you get physical symptoms; i.e., sweating, rapid heartbeat, dry mouth, need to go to the bathroom). So after your anxiety causes your body to become so wired, its response and reaction to this is to kind of shut down. Thats why you get a "not there" vegetable state feeling. This is actually a good sign, and know that your body's almost done reacting to the anxiety.
Even tho i absolutely hate this feeling, possibly worse than the anxiety itself, it helps to know theres a purpose and its doing some good!