Sounds to me like you have some sort of arrhythmia. I have recently been experiencing similar symptoms as you and am also 21, although drugs brought on my heart problems.
I have since quit drinking, partying, and smoking yet I still experience palpitations and bouts of tachycardia. I wore a holter monitor for 48 hours and while I cut the grass for 20 minutes my heart rate shot up to 170 BPM. The holter picked this up and my cardiologist diagnosed me with SVT or Supraventricular Tachycardia.
That's strange, you experienced symptoms yet they didn't pick it up? What did your heart rate reach when you had the holter?
I would recommend doing further testing such as a 30-day event monitor. You have a higher chance of catching the arrythmias during that time. It's very similar to the Holter but you were it for 30 days.
As stated above, I would stay away from any caffeine, alcohol, chocolate, or any stimulants as all of these can contribute to arrhythmias in those prone to them. Did they say whether you had a supraventricular tachycardia in the ER or a ventricular tachycardia. You need to be your own advocate with this and insist on further monitoring to get a proper diagnosis.
Unfortunately my insurance is not willing to pay for anymore cardiac Diagnostic testing. Nor does my cardiologist feel there is a need for any further testing because he felt that what ever it is he felt that it was going to pass with time that it was temporary.
I wondering if any of you thought that it might be worth my while to ask my GP to put me on a beta blocker or calcium blocker.