A little nervous :/

This is very strange to me but.. The last time I had sex was right after my period ended last month. For the couple times that I had sex that week we only didn't use a condom for maybe not even 5 minutes the first time. Now it has been a month and by this app it says that I should be starting my period today ( I didn't) and I have been having really bad heart burn for the past week, now I do know that I have been under an extreme amount of stress lately but I'm also nervous... What do yall think??