Hi and welcome to the site.
tests are very accurate. There are very rare reasons for a false positive. Plan B
will not impact the result of a pregnancy test. There are more often false negatives. This can be due to a test taken later in the day early in pregnancy. If a woman has been drinking a lot of water, the hormone level in the urine can be diluted and cause a false negative. First morning urine should be used for best accuracy. The other possibility is that there was a pregnancy but it didn't grow correctly. This would cause a miscarriage
. Sometimes there will not be any bleeding with a miscarriage but the pregnancy test will turn negative as the hormone level drops.
I hope this information has been helpful.