I would think it could be fluid (water weight), my theory is if you haven't been feeling that great then chances are you aren't as active as you would normally be - which would more than likely make you sweat less but at the same time you're probly drinking more fluids than usual because you get more thirsty especially if you run a fever occationally, and if you're like me you probly crave more salty foods. That's a good combination to cause your body to "soak" up more fluid weight, especially if you're not that sick that it is really putting you out of commission (if you understand what I'm trying to say) and causing your immune sytem and metabolism to really kick into high gear - which would cause the weight lost you're talking about that you're used to, but rather you just feel groggy and are funtioning a little slower but still able to keep up with the majority of your regular daily activities. That's my 2 cents anyway.