Yes that is disappointing! At least if you'd shed a few extra pounds there would have been a silver lining to being ill, I would feel the same. But as the other girls pointed out, it may catch up to having an extra big loss next week because you may be retaining water or something and once that goes you'll have lost a couple extra pounds.
This happened to a friend recently. She has been calorie counting for some time and then came down with a kidney infection. She ate almost nothing for 5 days and yet when she weighed herself she had lost precisely 4 ounces! The only conclusion we came to is that she literally had gone into a form of starvation mode where her body clung onto every ounce of fat it could to protect itself. A week later she weighed in and had lost 5 pounds so don't lose hope!!