Irregular periods

I'm 18 years old and for the last couple of years I had my period every couple of months I was on birth control but I can't be on it anymore because all forms of birth control trigger my severe migraines.
​   After I stopped they became regular once a month 5 day long periods. We'll last month I got my period about a week early i dont know why but I did.  And this month I had a 2 day light brown blood mixed in some discharge... But it was 2 weeks late for my period. Iv taken pregnancy test and they all come out negative but I still haven't started and I'm about a month late on my period... Help