I'm a little confused.

Kayla
So I'm on the patch. For anyone who doesn't know how it works you have three patches and you're in it for three weeks then you're off for a week which Id period week. For me I start my period two days after I take my patch off. Well, I had unprotected sex on July 6th. My period came on July 23rd and ended July 30th. I took my last patch off for the month on August 18th, so I was supposed to start on August 20th. I still haven't started my period and this app says I'm 4 days late. What do I do?!