Anyone have any advice....

I'm currently on day 38 of a very regular 31 day cycle.  Aunt Flo hasn't shown her ugly face as of yet, but I have now had two negative hpt, one on cd37 and cd 36.  Haven't had any Aunt Flo symptoms yet, feeling tired, have a slight backache, but not like a flo ache, and been having lingering cramping in my lower tummy.  Has anyone ever been in a similar situation?  What was the outcome, what should I do?