I read that normal healthy people can die suddenly from a heart arrhythmia - is that true, and should i be worried about it?

No: Very rarely, people who are otherwise healthy can have an unrecognized structural problem with the heart that can lead to fatal arrhythmias. How rarely? You are much more likely to die as a result of a drunk driver or a "run of the mill" heart attack. So don't worry, but do see your physician regularly so to increase the chances that "undiagnosed problems" will become both diagnosed and treated.

Yes and No: Almost every "young healthy person" who dies of an arrhythmia actually had a serious, but undiagnosed problem. And no, you shouldn't worry about it. We're talking near-lottery odds here of you being that person, especially at 46. Most problems that cause sudden death without obvious circumstances show themselves before now.