Okay so this might sound like a stupid question, but I woke up this morning with a little bit of blood on the sheet, only the size of your thumb print. Went to the bathroom to clean up and only a tiny bit came onto the tissue and it was brown. Thought it might of been AF (which isn't due till the 20th) but haven't had anything else all day, could this be implantation? Or would it be unlikely to happen overnight?