These two phrases mean the same thing and emphasize that the writer did not speak at all. AT: "I was completely silent" (See: [[rc://en/ta/man/translate/figs-doublet]])
Here "heart" represents the whole person. The anxious thoughts of the writer are spoken of as if they were a fire burning inside him. AT: "I became very anxious when I thought about these things" (See: [[rc://en/ta/man/translate/figs-synecdoche]] and [[rc://en/ta/man/translate/figs-metaphor]])