why do i think "whatever" ?

i'm a 15yo girl and i have the thought that i don't mind dying. it sometimes kicks in, but i actually feel the thoughts stronger recently. i have exam in the last few days but we only need to come to school if we have exam that day. in the middle of the exam, i suddenly gives in and actually didn't care about the exam anymore and leaves blanks everywhere. i know what i'm supposed to do but i didn't do it. normally i will finish my exam and anxious even if i study but that time i just felt very calm. after i get home, i try to clear my mind and starts to think about how the teachers and my parent will react when i have my results back. i don't mind them getting all worked up(but i normally do). just when i imagine myself saying "whatever" to them, i cried. i don't understand why do i even react this way. but only that particular word trigger my tears and the thoughts of dying just comes into my mind.