Most intravenous drips are saline and glucose. Depending on what the doctor ordered. Plus if you had surgery your body is more likely to retain water due to the "injury" your body is going through.

Normal body response to injuries is to retain water in the muscles to "act" like it's own ACE bandage.

I wouldn't fret over the gain. Give your body a chance to heal up, make sure you drink lots of water and eat well while it is repairing the damage.

Depending of if they put you totally under or not they may have inserted a catheter. That may explain the difficulty in urinating. This too shall pass.