Which prophet sent King Jehoram of Judah a letter warning of a disease of his bowels?

Answer: Elijah

A letter from Elijah the prophet condemned Jehoram's evil and foretold a severe disease of his bowels.

“Then a letter came to Jehoram from Elijah the prophet, which stated: “This is what the LORD, the God of your father David, says: ‘You have not walked in the ways of your father Jehoshaphat or of Asa king of Judah,”— 2 Chronicles 21:12 (BSB)