Hi NaLa, if you can afford it, would be good to go to a GP and have and EKG to start with. The doctor visit would be around $70 and might include the EKG.
Maybe tell the doctor that you would like to make payments as really need some personal attention with this matter. Im sure one doctor will see you about this.
I would also feel somewhat comfortable with the ER results. If you had some real problem they would admit you. So just relax.
With all the tests done, especially the EKG, you are honestly most likely just fine, with no cardiac issues whatsoever.
A couple of things make me say this: One, you are young and if you have no previous medical history of anything cardiac related you have no real concern, and two - a heart attack as far as I know does not cause a high heart rate, what probably causes the high HR during an actual myocardial infarction is due to high anxiety of the person knowing they are having one.
Just to clarify, an MI is caused by long term plaque build-up in your arteries, which can lead to coronary artery disease (a precursor to a heart attack) and the plaque itself is caused by too much bad cholesterol, hyperlipidemia (way too much fat in your blood) or hypercalcemia (too much calcium in your blood) and basically it just clogs you up from the inside, obviously not healthy. It takes a long time for plaque to build up anyway, but if you exercise, eat healthy and don't smoke or drink frequently, you really should have zero concern.
Also, when you were on the treadmill for your stress test, did you get any sudden onset of chest pain? That is one easy way they can check for any cardiovascular problems. If not, then the pains you were experiencing before must be non-cardiac, maybe they are related to one of your nerves. But this does not sound heart-related.
But really, you seem to be worrying way too much over this. I understand how the pains scare you, but you have been to the ER more than enough, your tests came back negative for anything organic, and the Xanex appears to help you. Your anxiety is probably contributing to the symptoms, especially the pounding heart. And no, a pounding heart is nothing to worry over. Whether you are exercising or really nervous or stressed out, your heart pounds the same way. It might be alarming when in bed, but it really is normal with palpable anxiety levels.
When you get really nervous, two things happen - your HR spikes because you naturally stress (your stress response) and your stroke volume increases, which is when your heart beat becomes far more apparent. Again, its normal. Take a breather, sit back and relax, you will be just fine. If you wish to get an echocardiogram, go ahead if it helps you put your mind at ease.
If you visit your GP they can refer you to a cardiologist who can perform the tests you feel you may need. I just had a holter monitor for a day and had an echocardiogram as well. All were clear. My father had heart problems since I was little so naturally when I had all of the same symptoms you're feeling I went into panic mode thinking the worst as well. I was told it was anxiety-sometimes you don't even notice your anxiety, but your body sure does. Visiting your GP would be a good idea though. Hope you feel better soon!