I feel the same way sometimes. The problem with depression is that the doctors can't measure it objectively like blood pressure or a bone break. Maybe when I feel depressed, I actually feel better than somebody else who claims to feel fine. I started writing down how I feel at different times this past week. When I reviewed it with my therapist, I was surprised at how moody I seem to be. I forget these things when I don't write them down. You might try keeping a diary or log.