Well, first of all I know that second hand marijuana smoke can not effect tests.
Is the test you took on the day your period was supposed to start (the 12th) the only one? Sometimes it can take about a week for hte hcg to show up. So maybe try again in a week or two.
I believe that if you start your period within 3+/- days of your expected start date you are considered normal. Also, I know long term pill use can cause many women to have lighter periods, or sometimes no periods at all. I am not sure if Nuvaring is the same?
At this point your doc might ask you to wait another month to see what your next period is like. But if you are worried you can request a blood test.
I would take a couple more tests at home. And given your past history of getting PG on birth control, I would give your doc a call and ask these questions. They are the only ones who can help and accuratley diagnose.