Look, a meteorite can land hit you as well, so what is there to be scared about?
Many apparently healthy people drop dead from all sorts of things each year. I had a former boss who for no apparent reason, dropped dead while in a hotel room bathroom while taking his shower. He was in good health. You can poke and prod and test and test from now until doomsday and when it's your time to go, it's your time to go.
You can get in your car and be driving down the road minding your own business and a semi truck can cross the median and hit you head on. Does that make you scared of driving?
Just have your mom do her tests..try to get her to get proper medical treatment and exercise and eat right. And hopefully she does not smoke. So that's pretty mucn all you can do. We do not live forever...not you, not me, not even our moms live forever (mine is 79). So don't be scared and fall into the media knee-jerk reaction of being petrified of suddenly dropping dead of a heart attack..