I'm not sure what the age of your boyfriend has to do with this but, yes, you could be pregnant. Nothing is 100% effective.
"Failure of sterilization techniques does occur. Sometimes after a Pomeroy tubal ligation the cut ends drift back together and heal, or the suture does not dissolve and the fallopian tube becomes connected again. Surgeons can provide information about their failure rate to patients on request, and patients should be aware that the risk of tubal pregnancy is increased in the event of a tubal ligation failure. The risks of failure also depend on the type of procedure used by the surgeon and patients should make sure that they are aware of the risks before they consent to the surgery."
What Is a Pomeroy Tubal Ligation?