Missed period

Patient: My period was due around the 15th of November but sometimes can come up to a week later, it kinda does what it wants to most months, but as of today Nov. 28th I still have not had it. I have all the symptoms of it, sore breasts, lower back pain and lower stomach pains but still nothing. My tubes were tied 13 years ago so Im not thinking pregnancy but the thought is there. I have not taken a pregnancy test for that reason. What could be causing this? It has happened to me before but it was about 10 years ago. Thanks, Anna