What must she do after she broke her fast on Ramadan 15th due to menstruation, then performed ghusl, then her period came again on Ramadan 21st and she completed her fasting without performing ghusl?

The minimum period of purity between two menstrual cycles, according to the Hanbalis, is thirteen days, and according to the majority of scholars, it is fifteen days. Recurrent blood is considered Istihada (non-menstrual vaginal bleeding) if the total duration of the blood, including the previous bleeding and the intervening period of purity, exceeds fifteen days. However, if the total duration is fifteen days or less, it is considered Hayd (menstruation). Based on this, the blood that appeared on Ramadan 21st is considered menstruation, and you should have stopped fasting and praying. You must now make up the fasting days of the menstrual period that preceded and followed the initial cessation of blood, as a menstruating woman makes up for fasting but not for prayer.