Why I find it difficult to breathe when meditating, a lack of oxygen in my head? And how to overcome it?

I just get started three days ago. When meditating, I felt hard to breathe and even the lack of oxygen to the brain. I felt my bell and chess were stiff and this intervene me in getting enough oxygen. My posture is right (I so sure about that). How do I overcome it? Should I keep going or find some changes?