Having very light brown cm only when I wipe for that 4 days. Yesterday it was the heaviest so I expected AF to come, but nothing still. Now my boobs are super sore and I have cramps. The brown cm is almost non existent now. Implantion or AF?

I used a pregmate HCG test and it was negative. Should I buy a first response? Or just wait and see if AF shows. Super confused!