If you are using birth control pills, condoms, and you used plan b, you are extremely protected.
As for plan b, you should not have really used it. It is for emergencies only, because the hormones aren't the best for your body. Birth control along with condoms is EXTREMELY effective. Even birth control pills alone is 99% effective whe ntaken right!
Being nauseated and having sore bbs is most likely from your birth control pills. Nausea can continue for up to the first 3 packs of pills, and if it continues after that you should talk to your doctor about switching brands or see whats up.
Being 40 days after having sex and having a negative pregnancy test, especially when using that much protection, means you are definitely not pregnant.