What Causes A Delayed Menstrual Period With Negative Pregnancy Test?

I am 37 years old, i had a tubal ligation 11 years ago...I am now 13 days late for my period that is always on schedule. I havent been sick but do feel a bit quesy.. I have 4 children now....is it possible that i may in fact be pregnant? I have taken a home test but showed negative..ty