Periods can be a tricky thing, and everyones is different. Sometimes our bodies do strange things, and can skip periods for no apparent reason. I personally would wait three months (thats with a negative pregnancy test) before I would go to the doctor. If you are really concerned you can always call your doctor and tell her your situation and see what she has to say.
I didn't get one lthis month. There's been a few months I've been with out a period. If nothing has changed in your life like your BCP or medicines etc. I'd wait till next month to see if you get a period.
With the new BCP I'm on it seems like I go every other month with out a period. As long as the pregnancy test stays negative I'd wait till the second month to see if you get a period before going to a doctor. But that's me.