With your age being 18 according to your profile, I would discount heart disease being the cause. Not even the best Doctor in the world would be able to give a conclusive diagnoses on a forum like this because there are too many variables and you need to be physically examined. Are there any patterns to these episodes? I mean, is there nothing you can think of that kick starts them? Do you lead a highly stressful life?
You could get an AliveCor monitor for your iPhone off Amazon. Then when you feel your heart acting up you can record your heartbeat. You don't need to be in the doctor's office simultaneously with an episode. It'll give you ECG read-out to show your doctor so he can visualize your heartbeat. Both the pattern and the speed of your heartbeat will tell your doctor a lot of initial information.
Don't grip the sensors too tightly because this can send the signal haywire.
In all seriousness, the proper answer is that you need to see a doctor. This is a peer support forum, and nothing you read here is a substitute for medical advice. The cause of your symptoms could either be a grave condition or not. Heart disease cannot be ruled out from your post. I hope you will take the time to get care. You need to be seen by a licensed medical doctor who actually examines you. Good luck.