Implantation bleeding or early AF

I am due to start my period on the 19th (Friday) and I have a 29 day cycle. I have started with bleeding today (3 days early) and I am NEVER early. It started with a few cramps and it’s bright red. There’s not much and it’s mainly when I wipe/a little in the toilet. Yesterday there was a lot of clear, watery discharge. So much that it came through my dress and onto the seat I was sat on which was embarrassing, I tested yesterday and got a BFN.

Anyone got any experience or explanation for this?