It happened to me last summer with an ear operation under general anaesthesia. After about 14 hours in the hospital with intravenous, I actually gained 10 pounds and couldn't stop gulping water.
Going home I was afraid to bump into anything for fear of bursting open like a pinata.
Just spend the next few days eating very little salt but drinking plenty of water. Your electrolytes will settle down to normal.
(If you have any friends with high blood pressure, and who doesn't
, you can ask them for a couple diuretics...like 2 days' worth)