Yep, I agree. It'll be not eating enough. Fatigue is when your energy levels can't keep up with day to day living, like you're tired to your bone. So even showering takes spoons.
I recommend a small, calorie dense snack mid morning and afternoon - 10 pistachios and a square of chocolate is mine (small so it doesn't trigger the nausea), or fruit. This stops you running on empty and means the fatigue can't get a proper hold. Never run on empty, it's not worth it.
Look up Spoon Theory. It's a chronic illness thing, very very useful for fatigue (I can't tell what's the drug and what's my baseline anyway).
As for the mechanism of why semaglutide causes fatigue... all drugs have side effects... I wonder if the new research today discusses it? I've not got the spoons to google and skim read it this week 🤣.