Late Period After Vasectomy & Negative Pregnancy Tests: What Could Be the Cause?

Question
My husband had a vasectomy in May '06 in July '06 he had the semen anaylsis done and it came back sterile. Of course we dont use any protection. My LMP was Jan. 31st I have taken 2 hpt and both negative. I'm pretty certain I'm NOT pregnant but, why would my period be so late.  I'm 33 years old and have never had a late period expect when I was pregnant with my daughter.  In Nov. I had a polyp removed from my cervix but, still had normal periods after that until now.

Any help would be great!!  Thanks  Melanie

Answer
a lot of things can make someone not have a period... any stress on the body, too much exercise, stress, an illness.  I would recheck a pregnancy test in 3 wks if you don't have another period, but it is almost unheard of for a validated vasectomy to fail later...