my period is 7 days late and I took a test last night and it came back negative what could it be

Any kind of dietary changes, stress, anxiety, etc...  can lead to irregular periods.  Once your body does adjust to the new dietary changes or other factors it will regulate again.  This is just a few examples of what could be happening without knowing more from you.  Also FYI  I have heard of many couples that were trying to get pregnant for long periods of time with no success even through fertility doctors spending 10's of thousands of dollars, but ended up easily getting pregnant when the couple stated eating Organic Live/Raw Vegan foods.  It is possible to eat a balanced vibrantly healthy diet through these foods.  The big factors are Vitamin B12, amino acids (spirulina/ vitamineral greens http://healthforce.com), as well as folic acid and essential fatty acids taht can all be found in veggies, fruits nuts and seeds.  When I refer to Live foods (Sprouted Live foods).  Any other questions that come up please feel free to contact me.

Cathi :0)