Help! Period 5 days late

Jessica
My period was due the 16th of may. On that day I wore a pad as precaution. Nothing at all during the day bit as it got later I found one small pinkish red dime size spot with cm on it. Nothing since then. My fertile window was the April 29th- May 3rd or 4th by Glow. Tested 17th got a BFN. Could it have been implantation bleeding on the 16th??