Had unprotected sex during my fertility week took plan b 40 hours later. is there a possibility of becoming pregnant anyways?

Yes: No method is entirely fool-proof. Wait 9 days from from unprotected sex and get a blood test.

Plan B One Step: is not 100% effective. Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes. Take pregnancy test if your period is > 7 days late.