Proper breathing techniques in relation to exercise?

I was taught one should breath in through the nose, expanding the stomach and then exhale through the mouth while contracting the stomach in towards the spine. My question is, should one do this while running or during other strenuous exercise? I've been trying it, and it seems to work just fine (possibly even helping a bit with oxygen flow), but also strains my stomach. I image that may simply be because my stomach muscles aren't used to expanding and contracting consciously and almost counter-intuitively, but I worry it might be the result of improper breathing technique. Can anyone shed some light on this for me? Thanks!