Period/trying to conceive

I went off my birth control in July.  When I was on bc I was deadset on a 28day cycle and since then I've been off the bc it's been 23-24day cycle.  This past month.. I got my period fri sept 18-20.. Then again on wed sept 23-25. This has never happened to me before.  I'm not sure when I should start counting from to see when in most fertile.