Because of his great love for them, Paul makes it clear that his rebuke in a previous letter to them caused him pain as well as pain to the church people in Corinth and the immoral man.
Possible meanings are 1) this is a metonym that refers to causing the Corinthians sorrow. Alternate translation: "in a way that would cause you sorrow" or 2) this refers to Paul's own sorrow. Alternate translation: "while I am sorrowful" [[rc://en/ta/man/jit/figs-metonymy]]