Why has my fitness level decreased over such a short period?

I did a beep test about 3 days ago from now, I easily achieved level 13. However, I did one today and struggled to even get level 11. Could it be something to do with diet? I have been reasonably active between tests. This is a slight concern as I need be at the peak of my fitness in a week. Any ideas?Why has my fitness level decreased over such a short period?Sounds like too much time wasted sitting in front of the computer instead of exercise.