I Am 16 And In Athletics. During An Intense Workout

I am 16 and in athletics. During an intense workout or sometimes during a football game, my heart suddenly starts racing and I feel my stomach tighten up and I get tired very fast. The more physical activity I do during this, the faster my heart is beating and the more tired I get. What is this? What can I do for this? Should I see a doctor? This has been happening on and off for many years.