The app says today is the 2nd day that

I’m late on my period, which I find really odd for me , as I usually ( from May) have it every month starting 1 day before the predicted date. I had PROTECTED sex with my my ex on the 15th aug ( he didin’t even finish inside me , even tho he wore a condom from start to finish ) and this morning I just took this test which to me seems negative, but I don’t know what to think anymore and I’m really scared. What could be the reason? Could it simply be the recent stress from the break up ? Should I test again or I shouldn’t have any worries of a pregnancy ?