Must I make up for the days of استحاضة (bleeding outside of the regular menstrual period) that followed the insertion of the IUD in Ramadan, knowing that it was like a menstrual period and I fasted during it?

If a woman's customary period changes due to an incidental factor, then any blood exceeding her usual custom is considered menses, as long as it does not exceed fifteen days. If the blood appears before its usual time, after the minimum period of purity has passed (13 days and nights according to the Hanbalis, 15 days and nights according to the Shafi'is and Malikis), it is also considered menses. If the blood appeared a short time after menstrual blood, such that the total of both bleeding periods and the period of purity between them is 15 days or less, then it is menses, and you must make up the days of fasting. However, if the total of the two bleeding periods along with the purity between them exceeds 15 days and nights, and the period of purity between them was less than the minimum purity between two menstrual cycles, then it is blood of istihada (non-menstrual bleeding), and your fasting during it is valid.