At times when relaxing, breathing is very hard and I get sharp pains which prevent me from breathing, what could it be?

Painful breathing. Could be a lot of things, like an infection, inflammation in the chest cavity (like pleuritis), musculoskeletal problems (pulled muscle or need for a chiropractor), pulmonary problems (like asthma, copd, or emphysema), cardiac problems, or even poor posture...And the list goes on! best advice is to go see your doctor and get it checked out.
Reading pain. When breeding is painful, and your body is telling you that something is wrong. It is for this reason that a simple evaluation with a family practitioner would be a good start. You have to be evaluated for infectious disease cardiac problem pulmonary problem and even as simple as arthritis of the rib. Don't hesitate on this get evaluated.