I'm Three Weeks Late For My Period And TTC. Pregnancy

I'm three weeks late for my period and TTC. Pregnancy tests are negative as of 9 days ago. Have sore nipples and other symptoms. On the day my period was due, I had cramping and pulling feeling, but no period. Why would all this be happening if I'm not pregnant? My periods have been regular with no missed periods all of my adult life. Any insight?