That is a very very small chance you got pregnant.
Keep in mind the plan b is likely to have some side effects. These include nausea
, headaches, back pain, bloating, cramps, fatigue, etc. It can also really mess up your period for a month or two- so don't be too freaked if you start experiencing these symptoms or your period arrives really early, lasts a while, doubles up, or is late.
If your period is more than several days late, take a pregnancy test with your first morning urine.