Period early?

I just went to the loo and wiped and there was stretchy dark yellow/brown CM. I’m CD 20 out of a 25 day cycle, I don’t normally spot I’m normally bang on, once I’ve noticed my period that’s it I’m on full flow. But this is weird?! I didn’t think implantation bleeding was a thing but could this be it?! Either way it’s period on her horrible way or something else!