Hi, Thanks for consulting us. The first differential diagnosis which is coming to my mind is an ectopic pregnancy. This is a life threatening condition unless treated timely. Although tubal ligation is considered a permanent method of sterilization but sometimes tubes get re-canalized & pregnancy gets stuck in tubes. Since you missed your period, pregnancy must be ruled out on an urgent basis by taking a blood pregnancy test & a pelvic ultrasound. Other possibilities are a normal pregnancy, or any other gynecological problem effecting your normal period pattern. Wish you good health.