my period is late

My period is four days late. And I've had unprotected sex but not for long and my boyfriend did not ejaculate at all. Could my period just be late due to stress? I also take birth control pills but I've been taking them for six months and I've never missed a period. But then again I just started tracking it. My last period was on December 20th, my boyfriend and I had sex on Christmas eve, December 24th. Also I haven't had any pregnancy symptoms no sore boobs, no implantation bleeding, no cramps. I've just been having really bad anxiety about possibly being pregnant. BUT I have taken two pregnancy tests and they both came back negative.