What Causes Spotting After Having Contraceptive Pill?

Hi, I stopped taking the pill after 14 years 2 months ago. I had what I believe is a normal withdrawal bleed on the 26th of November 2014 that lasted 4 days. Then nothing since. for the past three days now I have had light, what I guess would be called spotting of a dark brown discharge/blood that is quite minimal and not like a regular period. I have don't 3 preg tests over the last 2 weeks and all have been negative. Could I still be pregnant and should I be worried something is wrong? I understand that it can take a while for your body to get back to normal after stopping bc pills.