Every now and then my pulse shoots up rapidly and my heart beats so fast i begin to painc but after a few minutes i.m fine. whats happening to my body?

Go see your doctor!: Tell him/her what you've just noted. Your doc can check a simple ekg to look for abnormalities, draw blood to check your thyroid & chemistries and depending upon frequency of these episodes, can order either 24 hour holter monitor or an event (loop) recorder to look for what's going on when you have these episodes. In meantime, avoid caffeine & other (illegal) stimulants like energy drinks.