Listen my friend, there are many other people on here that feel just the same as you, and worse. As it happens, im am 15 and have had worse symptoms for about 5-6 months now. The symptoms I get are severe headaches, chest pain, palpitations, dizziness, and some others. I have had an ecg, ct scan, blood tests, and everything came back perfectly normal. Its called anxiety (or so I hope) and it sounds like you have got a similar thing to me. I have a fear of going outdoors incase I collapse etc and also dont realy like staying in my house alone in case I collapse. Chest pain, nausea and light-headedness are all classic symptoms of anxiety. Do you have panic attacks??
Heart problems are easily diagnosed and at 15 I would say its unlikely anyway. Have a look round the forum and you will see many other people who have similar symptoms. Dont think that you are the only one who has this because you most definatly are not.
Also anxiety cannot hurt you, so dont worry about it.
If you are still unhappy, see you gp.
Hope it helps,
james
p.S. Do you have msn messenger so that we could chat?? Me and you are going to get along great ;-)