irregular? help!

Rems • An unpredictable teenage dirtbag
so I've never really had a regular period and I've had mine for about a year (I'm older than thirteen so) ANYWAY period apps never get my period right and I can't find a pattern on any calendar myself. I have, however, always been underweight, I'm in the 21st percentile with my weight, height and age. I exercise a fair amount and eat good unintentionally... is there anything that you think could be causing my irregularity? 
(sometimes it's more than five or seven days off track so)

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