Breathing problems - Why does this happen?

I find myself holding my breath throughout the day for (seemingly) no reason at all. Right now, as I type, I'm feeling as though I need to think about breathing in and out rather than having it happen automatically. When I'm in bed at night, I'll feel a bit dizzy/light-headed and realise that I'm not breathing properly, and sometimes it gets to the point where I feel physically sick/nauseous before realising that it's because I'm not getting enough air. I've noticed that it's more frequent during stressful times (exams and problems at home etc.) but it happens when I'm totally relaxed, too. Oh, and when I *do* breathe, my airways don't feel blocked or tight at all, although I do suffer from hayfever so I, more often than not, have at least one nostril that's blocked at any given time. I know it's not good to deprive your body of oxygen which is why I'm wanting to address the problem ASAP. Is this some sort of disorder or syndrome of some sort, or? I'll definitely see a Doctor about it soon, but I don't know if there's much they can say about it to help me :-S