What Causes Delayed Periods And Negative Pregnancy Test?

Hi. I've been trying to concieve for 5 months after losing my son (neonatal death) my 1st cycle since he was born was 35 days, then since then they have been 28, I'm using clear blue fertility monitor which has indicated ovulation at day 12ish each month but last month I didn't get a peak and now no sign of my period. I was due on on Fri, did a test Sat which was negative. Any advice?