(a) Sayyidah Aishah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) said: "A woman sees something resembling blood after purity; what should she do?" He (peace and blessings be upon him) said: "It is a vein," or he said: "They are veins." (b) It is possible that what is meant by this is yellowness, or the blood that exceeds fifteen days.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1601