Took plan b, no period, negative pregnancy test. definitely not pregnant?

Pregnancy: Plan b is not full proof and has a strict timeline in relation to time of unprotected sex. Pregnancy tests are not 100% accurate and can be negative if tested to early as most detect a hormone in the urine which may have not reached detectable levels if tested before 7-10 days post conception, for most home pregnancy kits. I would suggest retesting to be sure.

This may be a side: effect from Plan B. Potential side effects w emergency contraception includes menstrual flow changes (spotting, bleeding, early or late period).