Heart palpitations??

im 16 and get heart palpitations every 2 weeks or so, usually it only lasts 3 or 4 mins but the thing i dont understand is, i get them outta no where, i wouldnt be drinkin alcohol,or smoking or running or no caffeine, id just be sittin down,it beats very fast and can feel it thumpin out of my chest and my throath always gets really tight aswell, why does this happen?