Hypotranemia occurs more in people with high sodium vs low sodium intakes.
Also people forget about how late nights thrash your adrenals and how important they are in regulating mineral levels.
Go watch an iron man event. Look in the nurses tent at the crew on salt drips. Ask em if they eat low sodium. 100% of em dont. Heck, just before they hit the deck they were drinking gatorade that was supplied on course! lol!
If you want more muscle and stamina, then eat more carb cals. Its that simple.
Get to bed early so you get full HGH release too.
I know teenage girls that eat more than you Alex. ;)
Ive also read that bodybuilders say the recovery time between sets should be based on when the room stops spinning.
You could also be going hypoglycemic from not eating enough carbs. Remember a grown orangutan eats around 8000cals a day and weighs similar.