If I did have implantation bleeding when should a test be accurate?

[Angel Of Death]

I am 11 days past ovulation, have been feeling some pregnancy symptoms and this morning I had just a little bit of brown discharge (sorry if tmi) not even enough to use a pantyliner. I usually do not get brown discharge before a period so this was a little bit different.

If this was implantaion bleeding when would a pregnancy test be accurate? I took one today but it came back negative, am I too early?