Be careful not to play the blame game. If you fell behind due to an unexpected circumstance or perhaps your boss piled on a few extra projects, that's okay. As you know, these things happen. Focus on what you did to make up time. Were you willing to work longer hours? Did you recruit extra help?
"I got behind on a project due when I got the flu, because I had to miss a couple days of work. I asked a coworker to handle a couple of tasks while I was sick so that when I came back to work I didn't miss any deadlines. I worked late for a few days in order to catch up."