What Causes Irregular Menstruation?

hey. my names amanda and im 17 years old. i just got my period back in june for the first time and it was really irregular and light for the past few months i wouldnt get it for five months at a time and then one month i had it for two days then it would go away and then come back again and so on. but now i have it and it is heavier then ever and i told my sister and she thinks im hemmoraging. that has me really scared and i wanna know if there are any other things that could be wrong