Scared

Okay so my last cycle was August 24-28 and I had sex September 5th, I was suppose to start my period yesterday on the 18th according to <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a>, but I haven't. Should I wait to see if I will start at the end of this month or take a pregnancy test ?