The sensation that your heart is beating wrong is called a "heart palpitation". It can be caused by any number of things, including heart defects that are completely harmless, dangerous heart defects, or yes, anxiety
. There is not a very good correlation between people thinking their heart is beating wrong and their heart actually having something serious wrong with it.
To determine what it is you need to get an EKG while your heart is beating wrong. Did you have this feeling when you got your last EKG? If not see if your doctor is available, or go to the ER and ask for an EKG. Doctors can't tell much unless they test you while your heart is malfunctioning.
If you did have this sensation while you were having your last EKG, I suspect you may be having panic attacks.
You will most likely not die.