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.
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.