Do you think I Could be Pregnant?

Hey. I stopped taking the birth control pill about a week before christmas, so it has been since christmas that I had my last period. My boyfriend and I have used the withdrawal method, but he is almost certain he has pulled out in plenty of time every time we have had . About 3 weeks ago I had what I think may have been breakthrough bleeding. It happened about three times in the space of three days, where I would find a browny coloured thicker blood, but definately not my period, when I went to the toilet. I am now around 2 weeks lae for my period, and took two pregnancy tests strait after the bleeding (then realised it was too early for them to be valid). I took another one yesterday, which came out negative, but that may have been because I didnt read the instructions properly, and didnt put the cap back on. Could I still be pregnant?(i am 19 years old by the way)