What Causes Inconsistencies In Periods?

hi um well here goes I have been wondering I had 2 periods in the same month the second one was about 7 days before it was supposed to be due and 26 days later I got my period again both this period and the one before it were up to 5 days long as opposed to my usual 8 and it was more on and off bleeding but it would be enough to fill an overnight pad each day should I be concerned? (I'm 19 by the way)