Fatigue and exhaustion is a very common issue with MS. They are not quite sure what causes it, and I've heard many theories. One that my neuro threw out there is that our bodies are working twice as hard trying to rewrite new pathways around the damage caused by demyelination. Being sick, stress, etc. can make it worse. Sometimes extreme fatigue can be a signal that a flare is coming on, sometimes not.

A check for any vitamin/mineral deficencies is a very good idea. My fatigue can get so bad that they have done bloodtest checks a few times.

I call what happens to us sometimes the "pay to play" program. If you push one day, then the next is a day of rest. If able to, break big projects into small ones. An example would be work on something for 15 minutes, break for 15-30, then repeat. It takes longer than it used to, but it helps to see something being accomplished.