What day of your cycle does your uterine lining begin to shed, if you aren't pregnant?

I am wondering if you do not conceive on what day of your cycle might the uterine lining start to shed (like how many days BEFORE the first day of your period does this process start?) My cycles are pretty regular 28 day cycles. I am TTC, and we did the BD on my fertile days this month - my period should start @ Nov. 9th and I'm wondering if the twinges I just felt could be implantation OR uterine lining shedding....thanks!