What Causes Delayed Periods And Negative Pregnancy Test?

I am now 7 days late (my period cycle is average 25 days) I am now on day 31, have experienced period like cramping but no period for about 4 days (very unusual as I only cramp the day I begin my cycle). I have been slightly dizzy and minor heartburn. I have taken 2 EPT pregnancy tests (one on 3 day missed and one today) both come up negative. My breast were very sore about 3 weeks ago, now not so much. Not sure what to think.