I don't know who in the world told you that if you didn't get a period you could have heart attack. NO WAY, NO HOW. So get calm. If that were the cae all perimenopausal women would be dead. You don't even want to hear what I have been going through and I'm almost 53.
YOU ARE NOT GOING TO HAVE A HEART ATTACK. You are still very young and your body just may not be adjusted as of yet. If your family has a doctor, if your worried go see them. Or talk to a adult you trust, Mom, Aunt, Whoever.
But not getting your period will not result in a heart attack. Wait until you get to my point in life. LOL
You will have plenty to worry about. But for you at 16 not for another 30 years. Take Care.