13 days late,Nagative Pregnancy Test

Hi i'm Aileen, 26 years old, from philippines. I'm 13 days late on my period right now and i have a drop of brown blood on my underwear, is it implantation bleeding? This past few days i have watery discharge, and i have lower backache till now. I already do 3x pregnancy test on my urine, but i've got negative.
Who else here got the same situation like mine, but ended up possitive? I'm just really confused. I really want to be pregnant..