What is the ruling on making up the days of Ramadan that I missed, after dark red stringy discharge appeared before my menstrual period, which I thought was menstruation, but then a doctor clarified that it was not related to my monthly cycle?

There is no sin upon you for what you did, as long as you did not intentionally violate the Sharia. You must make up the days you missed fasting, whether the blood was menstruation or Istihadah (non-menstrual vaginal bleeding). As for the blood itself: if you saw it after the minimum period of purity between two menstrual cycles (13 days according to the Hanbalis, 15 days according to the majority of scholars), and it carries the characteristics of menstrual blood, then it is menstruation. However, if you saw it before the minimum period of purity and it did not recur, then it is Istihadah.