Implantation bleeding but negative test

I have no idea what is going on. I had some very light spotting on CD 27 and CD 28, and took a test in CD 29 which was negative. I thought it might be implantation bleeding but wasn't sure if that usually happened earlier than one day before your period starts. Is it too early to be testing? It's driving me crazy!! I'm hoping someone can give me a little insight :)