What Causes Delay In Periods Despite Having Negative UPT?

I have been on birth control for over a year now and I take it anywhere between 930-11 every night, without missing one. Seven days ago me and my boyfriend had unprotected sex but he pulled out, this just happened to be day 2 of my fertile week. I am supposed to have my period in ten days, and I took a pregnancy test today and it came back negative. I know this could be false but can you tell me how likely it is that I am pregnant?