Cantiga 104 of the Cantigas de Santa Maria of Alfonso X

The Bleeding Host
A squire had a concubine who was very jealous of his wife. The concubine asked her neighbours for advice on how she could have the squire to herself.
The neighbours advised her to go to mass and steal a consecrated host to use as a love charm.
The concubine took communion, but did not swallow the wafer offered to her by the priest. When she left the church, she removed the wafer from her mouth and hid it in her headdress.
When she entered the town of Caldas de Reyes where she lived, blood began to run from her headdress. The people, seeing the blood, asked her who had wounded her.
She reached up and felt the blood and told them that God was punishing her. When she told them how the miracle had occurred, they praised God.
The woman returned to the church and confessed her sin, prostrating herself before a statue of the Virgin. Then she joined a convent.
Source: Arizona University

List of Cantigas de Santa Maria