Hyponatremia means the sodium concentration in your blood has dropped below the normal range. Sodium is the main electrolyte that keeps water balanced between your cells and bloodstream, so when it falls the effects are felt in the brain first: headache, nausea, confusion, and in severe cases seizures. It usually happens when the body loses salt (through vomiting, diarrhea, or heavy sweating) and only plain water is put back, or when someone drinks large volumes of water while eating very little.
On a GLP-1 the ingredients for it can line up: reduced food intake means less sodium coming in, GI side effects can drain it further, and "drink more water" advice can tip the balance if nothing else is replaced. For most people the fix is simple — replace fluid and electrolytes during vomiting or diarrhea rather than water alone, and seek care for confusion or persistent severe nausea. See electrolytes on a GLP-1 and hydration on a GLP-1.