Late period!

I had unprotected sex with a guy last week and he swears up and down I'd have nothing to worry about! I am now a day late on my period (according to this app) I took 2 pregnancy tests yesterday and both came back negative, but I read that you should take them 2 weeks after you had unprotected sex and it'll tell you if you're pregnant.. should I be worried? Or am I late because I've been stressed a lot more than normal this month??