Lack of the expected period (amenorrhea) usually indicates possible pregnancy but not always. There are other conditions that can also cause amenorrhea. The possibility for pregnancy, in your case, is very small because you use a very efficient contraceptive method (birth control pills) and your husband had a vasectomy which is also a very effective contraceptive method. You can do a pregnancy test to be sure. If the test is negative (not pregnant) you can wait for 2-3 months and then, if you do not get your period, you may want to consult your gynecologist. Beside pregnancy, amenorrhea can be also due to hypogonadism, hormonally active ovarian cyst, side effect of birth control pills, PCOS, etc.