Help!!

Okay. So I have been sexually active with my boyfriend for about 2 months now. We usually use condoms, but I will admit that we have had sex twice without using any protection. I have recently gone to a doctor and got put  on birth control, but I'm supposed to wait until the Sunday AFTER I get my period to start it. My period is a week late and usually it's right on time. The doctor ran a pregnancy test, and it came back negative. I also did a home pregnancy test and that came back negative too. Does anybody have any idea why my period might be late? I'm pretty certain I'm not pregnant but this is kind of worrying me!