Haven't had my period, but...

So. I'm 16 years old. My last "sexual interaction" with my boyfriend was July 27th. I didn't have sex with him. Just heavy making out, but things were touching, you know? So at first I was little paranoid of precum being on me and then him fingering me and getting it inside me. Then I saw that that was a super low possibility so I wasn't too worried. Well I was supposed to get my period on August 15th, and I didn't. My period is never this late, maybe by a day, but it's been longer than that....so I took two tests and both came out NEGATIVE (Thank goodness). I know stress can delay it, but what else can? I still haven't gotten it yet and today is the 3rd day without it. Any good advice? UPDATE!!!!!! I got my period late today! ❤