A 'whoops' on last day of my period?

My boyfriend and I had unprotected sex on August 7/the 6th day of my cycle (last day of my period)... We're 90% sure he didn't finish inside me, but it definitely snuck up on him and I'm worried a drop or two was inside me. My period is "5 days late" according to my period tracker app, but my cycles have ranged anywhere from 31 to 42 days in length since I started tracking in feb 2015. I'm on day 39 of my cycle and no period.... How much do I have to worry about? :(