It your heart doesn't settle down, I think the best thing to do is get a referral to an Electrophysiologist so that he can help you figure out if you have an arrhythmia issue. That usually means Holter monitoring for a month. It is easy to do and doesn't interfere with activities.
Depends what type of arithmia. As far as if it can be fatal or not. Yes they can happen out of nowhere. I have SVT, which is a type of arithmia. I can go a year or more with no symptoms, and the bam out of nowhere, heart rate hits 190's and i get classic symptoms of a heart attack, without actually having one. My cardiologists has put me through the ringer as far as tests go. And my heart is very healthy there's no disease everything comes back normal. So that's a good thing. Just really need to get checked out you never know what type of arrhythmia you have. If you have one at all until you go get it looked at. Like nowadays I take a beta blocker and that pretty much prevents those bad things from happening.