missed period?

Andrea • Mmm... Im a total antisocial insane looser (: I have seen Gerard Way on Oct.27.2015
According to the app my period was supposed to be here on the 18th but usually, it comes a day or two before that, now I'm 5 days late. I tested a day after .My missed period but I know for a fact i didn't do it right. I got a negative. But my period still isn't here. And yes I had unprotected sex. But it was a quickie. I just never miss my period and out of the blue it's not here. Should I go for another test or what should I do?