The spotting that you had sounds like breakthrough bleeding due to the Plan B. The first injection should be given in the first five days of a normal menstrual cycle (day one is the first day of your period). This will protect you from pregnancy immediately. But if you start on some other day, you will need to use additional contraception, eg condoms for the first seven days after having the injection.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.