With a start

Next even I myself was waiting to see Sumati. She came before dark. Spoke kind words just as he did the day before.
"Grandpa, you look sad, don't you? What is your illness, Grandpa?"
I remained silent. Why should I raise a tempest in the mind of such a little child telling her about my illness, I thought. But she did not let me go at that. Catching hold of my hand she said, "Grandpa, don't you want to tell me? I will not tell anyone, Grandpa..."
Then she herself must have felt that it was not right to worry the grownups. She changed the topic. "Grandpa, last night we had our prayers in common. I prayed that you might be cured soon. You will soon get well, Grandpa, and as soon as you get well I will take you to the convent holding your hand."
I raised my head with a start, and cast a glance on her face :
To the convent? I had not even suspected that such a pretty, innocent angel of a child was an orphan and was staying in the convent orphanage. Filled with pity, I kept looking at her face. Shyly Sumati lowered her head and said, "Grandpa, why do you look at me like that?"
"Where do you stay, my dear?" your parents ......?"
"No Grandpa. I stay in the convent." Her voice had weakened and her eyes were filled with tears.

Could you please explain to me what "with a start" means in this context?