Sayyidah Aishah (may Allah be pleased with her) says: I would owe fasts from Ramadan, and I was not able to make them up except in Sha'ban. Yahya said: This was because of her being occupied with the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8210