What Causes Delay In Period Despite Having Negative UPT?

i didnt get period this month,nd also i checked with pregnancy kit,it was negative.i was married in dec2013,in jan2014 nd march 2014 i had taken pil.i missed my period in dec 2014,,i had tests nd it was due to prolcatin levels,in feb i got my periods correctly....wat may be the reason for this