What Causes Pink/brown Vaginal Spotting While UPT Is Negative?

I have not had a period since Nov. Light pink/brown spotting for a day or so 1/27-1/28. 2 negative pregnancy tests on 2/3. At what point do i need to see a dr? If this was implantation bleeding, wouldn't i have symptoms aand/or a positive test by now? I stopped taking the pill in august and have only had periods in Oct & Nov