99% effective if taken correctly. Should I be worried?

Hi,
Welcome to ehealthforum,
Chances of healthy sperms being present in pre-cum/pre-ejaculatory fluid are higher after you have had a recent ejaculation. If you have not had urination before your next vaginal penetration (urination can flush off the residual sperms/semen present in the lumen), chances of pregnancy are a possibility especially if she was during her fertile days. If she has been regular with taking her birth control pills at scheduled time daily and have missed taking any active pill in between, she should continue to have the protection. Wait and watch for her next periods/withdrawal bleeding. Look out for development of any new symptoms if pregnancy is suspected. She should visit her gynecologist for further evaluation and assistance if she does not get her periods within 10 days of expected period dates, or gets abnormal bleeding.
Take care.