What Causes Delay In Menstruation Other Than Pregnancy?

I am 43 years old and began having irrigular periods 1 1/2 yrs. ago. Last year I was diag. w/Breast Cancer, I have been taking taxoxifen since November 2010, I haven't had a period since October. My breasts (nipples) are very sore and I was concerned that I could be pregnant, will taking tamoxifen affect the result of a home pregnancy test? I have taken 3, all negative but I'm unsure when I should take them as I have no idea when my period should be.