When i was child i fell on my back and my chest start closing and i can't breath for a while then i was ok. what was that &could it leave any damage?

Doubt it: Sudden events like that as a child, after which you were fine are not going to have hidden damage that appears years/decades later. If you are preoccupied with this health issue get a checkup. Once you are cleared medically consider finding a life coach to explore this health preoccupation. Sometimes they cover up a stress you cannot easily recognize or manage.