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?
This could happen if you change your breathing pattern hence cutting off oxygen. As long as you breathe naturally there will be no issues.
In meditation, you are not support to hold your breath but to test your lungs you could try hold your breath for 10 seconds. If you cough or get any discomfort then it is best to get in touch with a doctor.
When one fist starts meditating, one's mind is unused to the activity and may start to panic. When the mind panics, all sorts of weird threats and pains and bodily effects might appear; the mind is too attached to its own activity, and it starts to distort perception in order to give itself something to fuss over. This isn't about your breathing — breathing is something that will always 'just work', until the end of your days in this body — this is about your mind rushing around trying to fix things that aren't broken.
Relax, it will pass.
Remember, you're just sitting on a cushion, so the absolute worst that can happen is that you'll fall asleep and tip over like a character in a cartoon (and no, I've never seen that happen, so don't fret over that either). In a few days or weeks your mind will settle down and effects like this will go away.
It sounds like you are concentrating on too many things at once, friend.
Perfect posture in the beginning stages of any breathing exercises will encourage strain because it takes a lot of effort to maintain that posture. The energy you put into the posture and the meditation is taking energy away from the breath.
The reason why is because we only have so much conscious energy to focus in the beginning of any cultivation. So when a guide says to get comfortable, it means that you are going to exercise (whether it is the mind, heart or body). Being comfortable during practice also encourages more control of ourselves and better chances of gaining insight into the technique we are practising.
Meditation is not just action for the mind, it is applicable to the body and heart as well. Focus too much on one part of the whole will neglect what is left and lead to imbalance. Meditation is the harmony of discipline and relief.
Cultivate in harmony