Why is Ernesto shocked when he sees Don Pasquale’s bride?

Ernesto is shocked when he sees Don Pasquale’s bride because the woman, who is introduced as "Sofronia," is actually his own beloved, Norina.1

Ernesto had been unaware of the elaborate scheme orchestrated by Dr. Malatesta and Norina to trick Don Pasquale into a sham marriage. Because Malatesta had not yet informed Ernesto of the plan, Ernesto was devastated, believing he had lost his inheritance and his love, and he had arrived at the house intending to say a final goodbye to his uncle before going into exile. When he bursts into the room and is introduced to the bride, he is stunned to discover that the woman Don Pasquale is marrying is the very person he loves.21