Hi. I'm a 15-year-old male, Asian, and have a family history of hypertension. Recently, I have had complications with exercising.

Starting from about 6 months ago, I have the following symptoms during or after running:
-Palpitations and elevated heart rate just after I stop running.
-Dull aching in the back of my head when I exercise.
-Blurred vision after exercising*
-Extremely swollen legs after exercising (veins pulling out, unable to stand).
-Pain in my whole body hours after exercising (especially in the chest and back of my head).
-Irregular heartbeat during exercising*
*(My heart once stopped beating for a while, and I nearly blacked out).

And these symptoms not related to exercising:
-Frequent pain in the arms, accompanied by a weird feeling of flesh (or arteries/veins) spontaneously starting to throb.
-Nightmares, waking up finding my heart beating extremely fast (I'd say almost 200 BMP, if possible).
-Depressions.
-Fatigue during day (becoming less and less the last while).
-Mild Insomnia (family history).

I have been to a medical doctor a while ago. She only listened to my heart, did not take any tests, and said that nothing was wrong.
I have developed a sedentary lifestyle over the last 2 years or so. One or two years ago I always felt fatigued. Now I feel less and less fatigued during day.
I exercise roughly 3x a week (2x P.E. lessons, 1x running).
Sleep deprivation could also be a factor.

There's no doubt that I will visit my medical doctor soon, but the question is how to put forward my problem seriously. I had the impression that the last time she didn't really take me serious (after all, I'm just 15).
Also, I have developed two phobias. I am constantly afraid to exercise. How can I avoid blacking out after exercising? And how can I prevent nightmares and awaking every one and a half hour finding my heart pounding scarily fast?

What are possible explanations for these symptoms? How should I deal with them temporarily and in the long run?