Regular 27 day period but very light bleeding, almost light red/dark pink. what could be the problem? been on antibiotics for over a week.

Light period: Your history lists PCOD, which makes light menstrual periods more likely. If you were on antibiotics recently, they would not cause a light period, but the physiological stress from having an infection could change your menstrual flow this month. After infection resolves, menstrual periods are expected to return to the usual pattern.