Hello! My Periods Have Always Been Regular But I Was

Hello! My periods have always been regular but I was under norethisterone for upto three weeks in November 2015 for my wedding. This is the first time I have ever taken such a tablet and ever since my periods have been very irregular and most of the time I have no clue when my periods are due. Right now I haven't had my periods since March. Is that a side effect of the medicine? Is there anything I can do to make them regular. Please advise.
Thanks for answering!