My Cycle Usually Comes Every 40-42 Days. I Am Now

My cycle usually comes every 40-42 days. I am now on day 51, and still no cycle. i took a HPT on day 48, it was negative. I have no energy, but the time change could explain that ( I guess). I amnot on any birth control as we are trying to conceive. Not sure if it isrelated but last week I kept getting waves of nausea and heart burn. This has past now for the most part. Upper back (mid shoulder blade) pain. Any avise as to what could be causing all of this?