So you went one month and 3 days from August to September. So, if I am calculating correctly you maybe should start October 22. But you know your periods better than I do. I just went on a one month and 3 day calculation.
It is possible that you are about to start your period.
Implantation bleeding has not been proven. Read
this, however, it has happened, just not been proven in the medical sense. The link above will describe how what many think implantation bleeding is really occurs.
It could be that you are ever so slightly pregnant that the HCG hormone in your body has not yet built up enough to stop a period, only to slow it down and make it very light.
If nothing happens in the next week you should get a blood test done. In the meantime, treat yourself as if you are pregnant.
I hope all goes the way you want. Please keep us posted.