So for a while now i've been feeling dizzy when I stand up after sitting down for a while and like everything goes quiet and fuzzy and I go very dizzy for up to 40 seconds or so. I've blacked out or fainted once tho. I don't know what it is and not yet told my parents as I do not want them to worry, seeming as though just over a year ago my brother died of cancer. I am 14, 15 in august.