Unless it's caught on an EKG it's rather futile for anyone to list all the possible things that can cause the sensations you are describing. It's just a huge waste of your time and would make you even more anxious.
The root cause of all this might be your anxiety levels, address the root cause, not the symptoms. Focus on the root cause, the symptoms are distracting you from getting this resolved.
It's really important to get your finger *off* your pulse points, because this activity doesn't give you any useful information, and it can become an obsessive, time-consuming habit, like picking at scabs.
You're young, without a family history, and you had an EKG recently. Therefore, your heart is healthy.
As itood said, get the anxiety (and remember, anxiety is a medical disorder as real as diabetes) treated by a good shrink. Follow the advice he or she gives you, and you will be back to enjoying your life as you should be.
As long as you do not have pain, dizziness, shortness of breath...I would not worry. Checking your pulse does nothing. I know, bc I was/am a pulse checker. If you check it in your neck and feel fluttering, that could just be the muscles in that area moving. They can move with the slightest bit of tension. YOu are young and really should not be so worried about heart health. If you are, see your DR for reassurance. And like Achillea said, maybe a shrink to get your anxiety under control. Nothing to be ashamed of as I am currently looking into it also! It can only help! Good luck to you!