Missed period

Hey guys, so I really haven’t been TTC but I mean at the same time I’m hoping to pop up with two lines on my PT.. but anyways on November 15 was the last day I saw my period... I’m officially 13 days late and decided to take a PT.. as usual it came out negative... im so confused as to why im so late and the test is negative... anyone have a clue at to why I haven’t seen my period this month??