My family has a history of heart problems, so when something like this happens, I start to think of everything that could go wrong. I'm about to ask my mom if she could take me to the doctor because this is terrifying! I'm only 18.
Because you have been around people with a history of heart disease, you are right to be concerned that one day this could happen to you. You should go to your doctor and ask him whether or not at 18 you need to be concerned, how best you can avoid heart disease in the future, etc. etc. However once you have been and your fears have been allayed, then you need to stop worrying.
You see what you have picked up is a fear by association, a phobia. You have developed an unhealthy fear of this happening to you because you know it has happened to members of your family. Well you really need to drop that fear, because believe me it will stop you from living your life to the full if you don't. Life isn't a rehearsal, one day all our hearts will stop beating FACT ( in fact it is a miracle they are beating at all - the beating has got nothing to do with us, we don't influence that at all - ponder that!), and one day, one beat before yours does, the one thing that you will really want to be able to say to yourself is "I Lived!!"
So go to the doctors, get checked out, learn how best to protect yourself and your heart from future dis-ease and then LET GO of this worry, because it will only make it worse anyway, and live, live, live.
"I ask myself everyday - "If today were the last day of my life, would I be doing what I am doing today?" And whenever the answer has been "No" for too many days in a row, I know I need to change something.” - Steve Jobs