Welcome to EmpowHER and thank you for your question.
You are not pregnant. However, the synthetic hormone in Plan B appears to have upset your menstrual cycle. Changes to the menstrual cycle is a reported side effect of the pill. Some women start their next period earlier or later than normal.
I am concerned that you had spotting, a light menstrual flow followed by spotting. This sounds unusual to me. Contact your gynecologist for further advice.