Could Delayed Periods And Persistent Spotting Be Due To Being Off The Birth Control Pills?

I'm currently 16 days late for my period. I have been experiencing brownish/pinkish discharge for about a week now that comes and goes. I have no pregnancy symptoms and I've taken 4 pregnancy tests and all have been negative. I stopped taking birth control in December 2009 after 13 years of use and have had regular periods every month since then except for May (one week late) and of course June which I totally skipped. Could I possibly be pregnant or is my body just now adjusting to being off the birth control?