it could be your carbon dioxide, that will make you tired i had those symptoms, get your doc to get a check up to see if you are retaining to much carbon dioxide.
I get this too. I also have asthma and sometimes have a hard time concentrating, feeling like my brain doesn't have enough oxygen. I usually try to take long deep breaths, instead of short, fast ones. I also need to take a walk outside, or at least turn my chair facing the window and just concentrate on my breathing, to help refocus.