Hi, There are numerous reasons why your menstruation cycle can run late, Pregnancy and stress being the most common. The light periods can be from the Plan B or from early pregnancy.
Since you keep getting negative results, make sure you repeat the test in two weeks to make sure. You also should consider whether or not you've recently been sick, gained or lost a significant amount of weight, have been under excessive stress or have taken any new medications. All of these things can affect your menstrual cycle and cause you to have irregular periods. I would bring it up at your next doctors appointment, if you continue to miss cycles it is a good idea to set up an appointment with your doctor to determine the cause and begin any needed treatments. Walter
While some women do get accurate results within a week this is not the case all the time and many many women are unable to get a accurate result until they are at least two weeks late.
Yes many women experience late r spotty periods after taking the plan B while it is not a common thing it can occur.