Missed period

I had what I believe was a period about a month ago. My OB said this may happen because my little one is sleeping through the night. I am now 10 days late for my predicted period. We did have sex while I was ovulating accidentally. I know this because I took a test the next morning because I was nervous. I have taken two digital tests that were negative. Should I contact my ob/gyn or is this just normal for postpartum.