WebMD states that it is normal to have a lighter or heavier period after taking Plan B One-Step. Your period may also come earlier or later.
According to WebMD, if you go three weeks without a period after taking Plan B, you need to take a pregnancy test to ensure that you are not pregnant. If you are pregnant, taking Plan B does not terminate the pregnancy.
Other side effects include feeling tired, vomiting, dizziness and pain in the abdomen. If you experience vomiting within 2 hours after taking the medication, you need to contact a physician to determine if another dosage is needed, according to WebMD.