Your body is used to getting a lot of sugars and carbohydrates. Sugars and carbohydrates provide plenty of energy, and they provide it quickly because the body can directly (sugars) or almost directly (carbohydrates) burn the food for fuel.
Now you suddenly starve your body from this source of energy – from the very source of energy that has always been available, and to which your body has gotten used.
Your body will eventually use other sources of energy. There are ways in which your body can convert other biochemicals into glycogen and then into sugars to fuel the cells. But it will take time (at least two weeks, possibly longer) before all of your systems have made the transition.
During this time, you will experience a lack of energy – because that’s what it really is. Your old source of energy (sugars and carbohydrates) has dried up, a new source of energy must be used now, but the conversion process is not yet up to speed – thus an enormous lack of energy.
After a while, the other processes will be up to speed, and your body will create its own sugars rather than relying on your (now virtually non-existent) sugar intake. You will then have more energy than now.
You may, however, never experience the same feeling of strength and endurance you have grown used to. The new processes that your body must develop, even when they are finally fully up to speed, will never be able to provide the same amount of energy per unit of time as the old processes.
The headache, however, may be caused by something else. You may be having an electrolyte disbalance. If you drink too much water your electrolytes will be too low, if you eat or drink too many supplements your electrolytes will be too high. From your description I cannot judge whether your electrolyte intake is too high or too low, or even if the electrolytes are a likely source of the headache. You might experiment a bit to see what helps.