6 days late???

Hi, I'm new here. My period is now 6 days late and its freaking me out. I haven't gotten my period since late August. I had protected sex on the 12th of September and I didn't realise until I checked this app that it was during my fertile window. I had sex again a week later on the 19th but we used a condom and he didn't finish in me. Like I said we used a condom both times but still. Before I was scheduled to get my period I had been very moody and my nipples had been hurting which is normal for me but still no period. I've taken two pregnancy tests and they both said negative. I've been really stressed lately. Could the stress be why I'm late? What can I do about it?