Was my period implantation?

So,

I had a period (4 days, first two days heavy, as usual) from the 20th September

I then ran my first half marathon on the 13th October.

My period arrived two days EARLY on the 19th of October and first day was a medium bleed, then very light for a day, then spotting for a day. I put this down to the fact that my body had been through a lot with my half marathon, although I was running regularly.

Anyway, I took a pregnancy test on the 22nd or 23rd, just in case (I’m an ex POAS addict!)...negative.

I haven’t really thought about it until now. But my period was due today and no sign! All I have is heavy creamy CM. I’m sure it’s highly unlikely but could my October ‘period’ have been implantation??