When your heart is beating extra fast for no apparent reason, that is called tachycardia
You really need to see your doctor, since it could be serious. I have a friend who has it. She is on medication to control it and she is doing great!! She is also a young woman, so really....get it checked ASAP!!!
Here's a link to help you read more about tachycardia http://www.mayoclinic.org/tachycardia/symptoms.html
Hope this helps & God bless!!!