Thanks for the query.
According to your description the possibilities are:
-Hypomenorrhea: scanty flow during periods.
imbalance leading to irregular spotting, premenstrual spotting etc.
-Rarely in case of failed tubectomy there is possibility of pregnancy
So you wait for one more week, possibly by that time you will get your period.
If not consult gynecologist once and get examined to find out the possible cause and take treatment accordingly.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429