I have had the same thing happen to me for years now. It started when I was 16 and I am now 31. It is very scary and even though I have had all the tests and every one came out normal I still once in a while get freaked out.
Have you had any testing done ex. Ekg, ecg, halter, stress test, ultra sound. If not then go to your doctor and request them. These are the tests they gave me to rule out any acual heart problem. If you have and they came out normal then breath deep and relax. Try to find ways to help you relax on a daily basis. There are many, many people out there that live with this feeling so you are not alone.
i hope this helps and take care!
Tamara xx