That would be CO2. Shallow breathing, which is fast, doesn't get rid of the CO2 in the body. A way to deal with it is to breathe deeply and slowly in, then slowly out. the way I was shown was to count - in for 5, out for 7. The longer 'out' expels more of the CO2.
Somethign to do with using all your lungs, rather than just the top portion.
At least, that was the way it was explained to me.
