as i say in my signature:
MOST anxiety happens at the subconscious level. JUST because you don't feel consciously anxious or had a day or two of calm doesn't mean your mind & body are relaxed. It can take months of reduced anxiety before a body goes back to a more non-reactive state.
the trainer is correct. it is muscle fatigue. I will take it a step further and talk about your anxiety. anxiety/stress cause a body to pour a lot of adrenaline/cortisol---the fight/flight chemicals. this is, in part, the cause of symptoms. Now as you may well imagine, our body always feeling like it is being chased by a bear wears a body out. IMO and my experience, anxious people are much more likely to have exaggerated muscle fatigue or tremors. so I am not the least bit surprised to read your post. It makes sense to me.
don't go all ALS here. you are barking up the wrong tree.