What Causes Irregular Periods?

hi im 17 and i was wondering ok this is i think my second time having an irregular period my first was when i was younger but this one it start off heavy for two days then the next day light and on and off it ended yesterday but i have a regular five day period my mom friend say its because my body changing and i ask a friend and she going through the same thing well went through anyway and today i feel kind of sick a little but not really and she said she did to she says it normal but i was jus curious is it