What Causes Palpitations And Discomfort In Chest?

My heart will randomly start beating extremely hard such as just walking threw a store or litterly just sitting doing nothing. But when it happens it hurts not bad but really uncomfortable, and a lot of pressure, spesially in my throat. I get really dizzy when this happens and if I don t sit amidiatly I will pass out, everything goes kinda white when it happens. This has been going on most of my life and becoming an issue. I m 25 and very active, I would greatly appreciate any help