What could this weird heart beat thumping be?

If you are really concerned and you want to be certain that nothing is going on then you can speak to your doctor about a Holter Monitor, where they hook up a heart monitor and you wear it for 24 hours (it is very portable and is hardly noticeable). The monitor will record all the beats of your heart and it will determine if you have any irregular heart beats. Another test that could be performed is an echocardiogram, where they do an ultrasound of your heart to look for any abnormalities. If your heart rate is suddenly changing on a regular basis then it would be a good idea to get it further checked out. Chances are low that you will suddenly die or experience cardiac arrest. You should aim higher and try for at least 70 or 80.