Hi Doctor , Im 21 Year Old Male , Smoking

hi doctor , im 21 year old male , smoking tobacco from when i was 15years old. Smoking marijuana every day (joint mixed tobacco+marijuana) sometimes i smoked 15-30 joints a day , but few last years im smoking like 1-2 joints a day , i drinked alcohol occasionally thru the years , im sitting allot playing computer for hours etc. Today i smoked cigarette and i get up from sitting and i noticed my heart started Racing like wild ,i put my arm on my left chest area and i felt Hard Pounding. It didnt stop for like 5-7mins and i walked i was total covered with sweat , i took my shirt off and felt better for like 10seconds then i was afraid and heart started racing faster and harder i felt so afraid and started walking again i didnt know what to do because i was in public place filled with people+all those people watching me made it worse for me , i came to the place where no one was and sit down for a bit , then i took of my sneakers and i tried to think about something else and somehow it started to calm , it was like 15-20 minutes i felt like im going to die , i can remember there was no pain only heart like i felt it with my breath i cant explain that , i wuld inhale and it felt like its affecting heart i dont know , my throat felt total how to call that - dehidrated throat - i culdnt swallow my saliva because my throat was dry yeah thats the word,breathing - i tried to calm my self down i expirienced this few times thru the years , so i breathed normal and when i sit and calm my self down it calmed down 100% but not imidiately ... i thinked im gonna die 100% please help me PLEASE tell me what is that