no period

haven't had my period in 2 months now I took a pregnancy test last month while I was supposed to be on it and I took one a few days before this months missed period. both were negative. I can't think of any reason why I would have two missed periods in a row. for the passed few years I go months without a period and when I do have a period it lasts about a month or less. this isn't normal I used to be regular.