Lately he had been having doubts about the Real Presence of Jesus in the Eucharist. Was the bread and wine really transformed into the Body and Blood of Jesus?
He said Mass at the Church of St. Christina. As Peter raised the Host and consecrated it blood started flowing out of it, onto his hands and the altar. God was telling him (and us) that yes, He is truly present in the host.
Peter was confused and tried to hide the blood.