I had this very commonly when I was young. It may have been related to the malnutrition of Celiac disease, which I did not know I had.
This happened to me. What it was is just an irregular period. It started about 2-3 days after the brown spotting stopped.
The dr said it was due to stress and me gaining weight.
But it possibly could be implant bleeding if you have been sexually active and not used protection. I would wait a couple of days and take another test and if it comes back negative perhaps you should go to the dr and get some blood work.