False negative?

Im not sure if you remember me from my last post but after having light pink flow, it eventually got a little darker and a littler heavier but then id go hours without bleeding then id bleed a little and yesterday i noticed that it turned brown so i decided to test last night before i went out with my friend for a drink, just to be on the safe side. The test said "Not Pregnant" so then i woke up this morning and had a brown spot. Moral of the story is , in still spotting and now its turning brown . How common is it to get a false negative ? Not to mention i was supposed to start my AT yesterday . I started spotting about 3 days ago. How many women have experienced this ?