Hmmm... well, I've been down to the point that I have 0 motivation and even looking at pictures in a magazine are too much for me. My body does feel heavy at that time. I can usually walk and do things, it's just I feel weighed down and don't want to.
So, I guess the question is if you're just feeling completely blah or if you actually can't make your body go. That would be the red flag, is if you can't move even if you want to.
I'm sorry you feel so terrible.