Can I be pregnant? Please read the additional info.?

I was on birth control for 2 weeks, but started to have breakthrough bleeding that lasted a week and a half and the doctor told me to stop using them and immediately I stopped bleeding... I had unprotected while on the pill and I am now 12 days late for my period, I have taken several pregnancy tests and they come back negative, can I still be pregnant?? Why don't I get my period?