Irregular, really painful. what do i do?

i`m 17. i had my first period when i was 12. ever since, it visits me irregularly. like, 3 to 6 months interval. and when it does, it`s really painful.,up to the point wherein a can`t do anything. i can`t stand the pain, so i take pain relievers. it gets more painful every time. i had my last period 4 months ago, i think. i`m not having though. so, what could possibly be wrong with me? what do i do??