I went off my BC pills in August, I got my period right away (4 days). September came, I was 2 days late (2 day period). October came I was 4 days late (2 day period), it's now November I'm 12 days late. I have taken HPT and both stated negative.
My sister took HPT for 3 months each time and it was several times in the 3 months that she tested, all came back Negative. She was so fed up with her period not being present, she went to the doctors and the ONLY thing that could determine her pregnancy, was a vaginal ultrasound...
It all depends on the person, I would check with your OBGYN.
The strange thing about my situation is that I got my period after I stopped taking my pills (5-7 days of active pills before the end of the pack). My October period was 2-3 days longer than usual (I spotted for the last 3 days). Now I am over 2 weeks late for my November period. I have stopped taking BC before and I was never this late for AF..NEVER.
It's not uncommon to miss a period after going off birth control, but it is rather ununsual to get pregnant so quickly. I agree with your sister-in-law actually... your body is readjusting to controlling your menstrual cyle (while on birth control, it is regulated for you), so just give it a couple more weeks. You may not have your period for another month! When I went off the pill, I had similar symptoms, but after a couple of months my body had taken over and I was back to "normal." It was like 5 months before I got pregnant!
Take some deep breaths and try to watch what goes into your body in the meantime. If you are pregnant, you don't want to hurt the little one!