How do you change this sentence into indirect speech? <br/><br/>" He was going on about the difficulties faced in the project when I walked into the room, " she said.<br/><br/>My answer is - <br/><br/>She said that he had been going on about the difficulties faced in the project when she walked into the room.<br/><br/>However, I have difficulty explaining why I use "walked" instead of "had walked", given the rule of moving a tense back when reporting speech.<br/><br/>Thanks.