What is the ruling on the prayer of one whose clothes were stained with menstrual blood and who forgot to change them before prayer?

Your prayer is valid and there is nothing upon you, because whoever prays with an impurity that he is unaware of, or was aware of it and then forgot it, his prayer is valid according to the preponderant opinion of scholars. This is what Al-Nawawi chose and attributed to the majority of scholars. Sheikh Ibn 'Uthaymeen also chose it, basing his argument on the verse: (Allah does not burden a soul beyond its capacity. It will have [the consequence of] what it has gained, and it will bear [the consequence of] what it has earned. Our Lord, do not impose blame upon us if we have forgotten or erred), and on the action of the Prophet (peace and blessings be upon him) when he prayed with his sandals while there was filth on them, and he did not repeat the prayer.