How early can implantation bleeding occur?

Khai • All ready for my rainbow baby !
Hi all, I had light brown discharge and a tinge of blood (upon further investigation) on CD21. AF isn't due for another two weeks thereabouts (35-day cycle). Thus I feel it's too early for implantation bleeding to have been possible, and am wondering if any of you have thoughts / experience on this?
My cycles are 4 and 5-week alternates and this would be my 5-week cycle. AF arrived 15 Oct and lasted 8 days. 
Any inputs would be greatly appreciated. Thank you 😊