What Causes Missing Periods And Negative Pregnancy Tests?

I am 8 days flate for my period. Have done 3 clearblue preg test and all are neg. I have never missed a cycle in 19 years. Always regular 30-32 days. Only time I have been late is when pregnant (both miscarried). Believed to have had implantation bleeding, slight nausea, tenderness, food craving and fatigue. So confused. Did a blood and urine test today. What do you think?

Nadia