When should I test?

My period was due the 26th and I still haven't gotten it and I'm not having the usual symptoms I typically get before it's due. I had sex last month during my period (27th ) and on the 17th of this month (this app says I was fertile that day. Used the pullout method I assume he did it right I've never gotten pregnant before off this method. Just wondering if there's a possibility or could my period just be late