I took my last BCP on July 20 and had my regular what they might call withdrawal bleed on July 24, my next cycle came as usual 28 days later August 21 my third cycle was due on September 18. I am now 6 days late and on day 35 of my cycle. I took one pregnancy test the day after my missed cycle and my husband and I were sure it was a faint positive then took a second two days later and it was negative and still not sign of my period. Is it possible that I have post pill amenorrhea or should I continue to wait for my cycle to arrive or to test again?

Wait it out. It's only been about week. If you start getting symptoms like nausea, headaches, constipation, sore boobs, then you should worry. But right now your period is just getting back track. If it becomes too much of a concern then visit the dr

Tests prior to 14 days late, often show up as negative, even if false. So just wait it out and test. Make sure you test only first thing in the morning with the first urine of the day