Red spotting six days before AF due.

LJ • Baby girl born Feb 19, 2015 and second girl born Jan 20, 2017. Hoping to convince my husband to try for a third!
Could this be implantation bleeding or is AF just threatening to start early?  CD 21 of a 26 day cycle. Around 8 DPO. I haven't used OPKs so I don't know exactly which day I ovulated just going by the symptoms I had then.