You may think it is as simple as the fact that when you are tired or feeling ill, you rest up more, move less, and you comfort-eat to make you feel better. And to some extent, you are right. These things will make a difference.
But, one of the biggest reasons is that when you are tired or ill, your body starts to fight to make you feel better. This is where your sympathetic nervous system gets jacked up, and responds by releasing hormones that get you going and try to give you more energy.
This is fine in the short term, but over days and possibly even a week or so, it can leave your body in a highly stressed state due to the constant high levels of these hormones.
We know that the body's response to stress is inflammation, and the body hates inflammation. This is when your body stops working as well as it should so that it can fight the inflammation. When this happens, the processes in the body aren't working at full capacity, and your weight-loss can start to slow down.
So what the best thing to do?
Depending on the severity of your tiredness or illness, training may or may not be a good idea. You will need to listen to your body. But if you are training, try to keep the volume (number of reps and sets that are programmed) as they are because we know that volume is key to what we want to achieve. Instead, try to take the intensity and load down, take longer rest periods, and reduce the weight that you are lifting.
These simple things will help you to keep moving forward and will help you to recover. Add in the recovery strategies that you are all (hopefully) doing (the deep breathing, the sleep routines and the following the nutritional guidelines), and you will soon be fighting fit, and ready to push again.