What Causes Delay In Periods With Negative Pregnancy Test?

6-8 days late with period. I have taken 4 pregnancy test. 1 test 2 days late negative, 1 test 3 days late using first mornin urine negative, 1 test at 4 days late negative, and one at 6 days late negative. I haven't been experiencing symptoms such as nausea, no breast tenderness. But I am very nervous. I have butterflies in my stomach, pounding heart and the lump in my throat feeling. I just don't understand why I haven't started. I was thinking after 4 test all negative why I am still worried