Increased heart rate, strong pounding sensation

Yesterday I had a very scary experience. First of all, I'm a 16 year old male. As I was walking home from a friends house, I started getting very hot as I was bundled up in a coat while it was rather mild outside, walking home at a brisk pace. When I got home, I felt an unusally fast beat, much faster than i had ever heard or even felt before. I couldn't believe that this was my heart, but it was. While standing up, sitting, or laying down I could clearly not only hear but FEEL my heart pounding, it was so strong that I could feel the beat through a longsleeve and a very thick sweater.

Upon checking my heart rate, I found it was 23 per 10 seconds, or 23*6=138. There was nothing I could do to slow the rate down, I tried splashing my face with cold water, I tried drinking lots of water, I had absolutely no appetite and felt extremely weak. Finally, scared as hell, I just lied down on my bed and listened to my heart beating faster than two times a second, thinking I was going to have a heart attack. My heart rate finally started slowing down A TINY BIT to 120 bpm in around an hour, the entire time laying on my bed. My RHR continued steadily down until reaching 100 bpm before going to bed.

This morning I got up very quickly and took a warm shower, and i realized my heart completely racing again, up to around 140 bpm. I panicked and jumped out, and it took around 5-10 minutes for my heart rate to return to 90 bpm. Now checking my heart rate I'm at a constant 90 bpm resting, but whenever I get up or even walk down the stairs it quickly accelerates to 110+.

What's wrong with me? Should I see a doctor? Is this normal? Why is my heart so audible to me when its pumping fast? What started the acceleration in the first place? What is the limit at which your heart can operate in bpm, and for how long? Was it dangerous that I was at 140 bpm for almost an hour?

[xstalkrx]

your heart rate ... at 16 years of age... can go up to around 204 beats per minute.

[xstalkrx]

please give us an update on your condition

[pamelasmc]

Research "orthostatic intolerance"

[Jennifer000]

I am 21 years old. I have had a problem with anxiety most of my life, but never had any sign of heart trouble. Then in my late teens I started having a fast heart rate. It took over my life. I was afraid to do anything to speed up my heart because I was afraid it would give out on me. When I entered my twenties the fast heart rate only got worse and now it pounds so bad, it tend to keep me up at night.
I went from being a normal healthy girl to an unhealthy emotional wreck.
I have seen a cardiologist and he says there is no problem. However, I beg to differ.
I'm at a loss and don't know where to turn. If you have any advice for me, it would be greatly appreciated. If not, please pray for me.
Thanks.

[Jennifer000]

to it's live. I thing you should see a heart docter liv. It couldn't hurt. I'll keep you in my  prayers.