It’s not “diet” energy drinks.
I’ll break this down someday simply. What carbohydrates are good at is replenishment - literally everything your burns requires energy, and the most efficient source of that to begin with and replenish it is carbohydrates. When it runs out of carbs, it uses fat, which is only slightly less efficient than carbs.
However, that slightly smaller amount of energy your body gets? It sends signals to your brain that it’s not getting as much. So it uses two very important compounds in greater abundance - sodium chloride (salt) and potassium (and to a smaller degree, magnesium).
The long and short? Drink more water and make sure you’re either eating enough potassium-rich foods or toss a bit of salt into your water if you don’t eat cold cuts regularly. It’s almost definitely to do with the sudden deprivation of water retention with lack of carbs.