That could be. For now, I would live as though you are pregnant just to be on the safe side. Test weekly or bi-weekly. I would say if it's been over two weeks since your last time having unprotected sex and there is still no positive test, you should go to your doctor to get tested for hormone levels outside of HCG.
I would also like to add that my period is always on time. This time i have not gotten it and it has already been a week. I have taken a few tests and they all came back negative. The only problem is that I dont not know what day I could have gotten pregnant and that could be why I am late and I am not getting a positive test right away.
You could be pregnant, if you did not use a condom during and since being on birth control. Birth control takes at least a week (sometimes a month) of consistent use to protect a woman from pregnancy. You only took it four days, so you were never protected.