Is it necessary to perform ghusl upon seeing a small spot of blood on the seventh day after heavy bleeding ceased on the sixth day, or upon the complete cessation of blood?

When blood is seen, a woman is considered to be menstruating until the blood ceases, along with any yellowish or brownish discharge associated with it. Purity is attained by observing dryness (a cotton swab that comes out clean) or the white discharge (al-qassah al-bayda'). If there is doubt about purity, the default is that menstruation persists. After certainty of purity, one must promptly perform ghusl (ritual bath). After ghusl, if something that is doubted to be blood is seen, the default is that purity persists, and no attention should be paid to it. The questioner's action is correct. One must be cautious of obsessive thoughts (waswas) and not give in to them, and perform ghusl in a normal manner by drenching the entire body with water.