There are a few reasons that it could be coming irregularly, first off did you just start getting your period? Mine took about a year to become regular. Next are you very athletic? Being very active does make it so that you can have less regular periods. Lastly sometimes if you don't weigh that much, maybe 85ish pounds that could be the issue because if I'm correct you need a certain amount of fat to menstruate. Hope this helps.
