You did follow the directions properly? And really..WTF are you doing not using a rubber. If you INSIST he will. If he won't...he doesn't respect you or your feelings & wishes. Later it be your hopes and dreams he $hits on.
Plan B is a backup plan for when your other protection method(s) fail (like a broken condom). No birth control is 100% except abstaining. So, the answer is yes, you can still get pregnant, but it is highly unlikely, especially since you took it 20 minutes afterwards.