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It does sound like it could be potassium. Eating carbs raises glucose and insulin - glucose + insulin is actually given to patients with a high potassium level in order to lower it - perhaps the same thing is happening naturally with you to make a normal or low level lower.
If you check out medically, you might want to increase potassium intake, especially when you carb up. Ask your doctor first, since too much potassium can be dangerous.