Period?

Okay. I am currently on the mini pill for birth control and my period returned after I was about 5 weeks postpartum. (august)I'm currently 13 weeks postpartum. It seemed pretty regular like before. I spotted a little last month too. My period for September was due about 4-5 days ago. I still haven't gotten it yet, or any signs that I usually get before it. I have been unusually nauseous too. I did forget to take a pill early last week and I also happened to have had sex on that same day before I was due for my pill. we usually withdraw along with birth control and breast feeding, but he did cum a little too quick before withdrawing.... I took a pregnancy test like 2 days later, but it came up negative. What are the chances I could be pregnant again? When do you suggest testing again? Tia.