I just googled it & found this:
A recent study has estimated that 143 women in 10,000 get pregnant after a tubal ligation, making the percentage about 1.43% which is better coverage than you’ll likely get from other forms of birth control. In these rare cases, the fallopian tubes often grow back together, allowing the egg to come down and be fertilized and then move into the uterus. Many of these pregnancies actually turn out to be ectopic pregnancies because of the damage done to the tube during a tubal ligation.
Definitely see your doctor if you don't get your period, just to be sure!