last thursday and again today I experienced,out of know where, my heart started racing,followed by horrible sweating, hands shaking, dizziness,head spinning , and nausiness, lasting for about 30 minutes, then slowly began to calm down. The same exact thing happened exactly one week ago today but today I lasted about 30 minutes compared to 20 minutes last week. I have no known medical problems that I'm aware of, healthy my entire life and now this has occurred and I'm scared of what it may be. Last week I thought maybe I needed to eat something or drink something cold , which seemed to help, but now the exact same happened today...out of know where..and I'm really concerned. I'm 52 years of age, smoker, non-alcoholic drinker, and 5ft4 135 lbs. Any idea what might be the caused