Brown discharge 🙄 TMI PIC

Sarah-Jane • TTC 16 months💏We did it🤰🏽Sophie born 10/7/18💕
After 10 months TTC and one Chemical I'm a bit confused ... past 5 cycles (I'm v regular 27-28 days each month and O day 13-14 each cycle) I've started spotting brown (lightish brown to darker mushy brown towards AF) from anywhere from 7dpo - 10dpo until my bleeding starts, now I've had thyroid tests, PID, transabdominal scan, 21 day blood tests, hormonal blood tests, all up to date with smears and STI checks
I've had my scan today and no issues anywhere no endo, blockages, cycts etc the docs are stumped as to why I am spotting so early. I've started b6 and b12 in my pre natal vits this month.
Anyone got any ideas at all, or even happened to them? I'm SO confused 😕 this is what the spotting looks like from the day it starts to a bit darker and a tiny bit more until AF