12 DPO and brown thick cm?

I'm 12 DPO and I have excessive CM but today I noticed when I wiped it had a light brown tint to it. I ovulated on Nov26th and BD on the 25th and 26th I usually have a 28 day cycle that is like clockwork every month. I took two HPT and got a BFN both times AF is due between 2morrow and Friday I'm just wondering if this is implantation bleeding or AF any advice would help! The only symptoms I've had is extreme sleepiness, nausea (but only once) and swollen