I would go to a Dr and talk to a Dr about it because it may be that your heart is having to pump harder than then adverage pump of heart beat.
My question to you is what prompted you want to check your heart beating in this way, were your experianceing strange sensations in your chest or something???
Try not to worry as stress can cause strange things to happen as well but it would be something I would want to get an answer to my self from a Dr. You may need a stress test if a normal ECG shows nothing.
Forgot to add also, When i feel my pulse in my kneck or wrist it feels fairly normal. Only in my chest under my left arm does it feel very fast and irregular. I cant feel a thing under my right arm. Should of put this in the main question, What are your views on this? :O
You could be experiencing a high heartrate right now because you're pushing your body too hard - or you're lacking important nutrients? (vitamins, minerals, protein, carbs, fats, etc)
Or maybe a combination of the two?
The only way to know is by visiting a General Doctor (or cardiologist) with an ECG machine.
If your heart is behaving funky while he is taking the ECG, he will be able to see whether it's necessary to send you to a cardiologist or not.
If your heart is now beating +-140bpm at rest and you don't feel pain in chest or shortness of breath, then you're not in immediate danger..... but a high heartrate like that isn't healthy either so it must go back down soon or later.
If your heart is beating +-180bpm at rest or higher then I guess it's worth calling a general doctor and ask them if it's necessary to get it checked out immediately, just so they could give you a pill to calm it down or something.
The rates 140 and 180bpm are my estimations which i guess are quite correct.
Maybe other people will be able to give a more correct number though!