Could someone over hydrate?

Yes: There is something called water intoxication which throws your electrolytes into imbalance and makes you sick.

Normally, No: To over hydrate is to add excess water to your body. The normal kidney does well to prevent this from happening. But there are other conditions which mimic this excess water state; eg. Illness effecting salt metabolism or salt intake. These could present in the same manner as water excess. It is important to know if your kidney and endocrine functions are normal with a periodic health screen.