Why can't i stop thinking about dying?

not like i want to die, but that i fear it SO MUCH. on july 4th they had planes fly over our city and i thought they were going to crash into my house, ever since then anytime i hear a loud vehicle noise outside i think somehow it's going to kill me. i've been able to calm it down the past few days but all the new stuff about north korea is making my fear rise again. any sort of hum, beep, NYOOM from the outside makes my heart jump. why is this happening and can i stop it?