Yes, urinary retention can indeed be a side effect from hydro. Drink a LOT of water, cranberry juice is excellent also. Try a mild diuretic also. Go homeopathic first-look for one at at your local vitamin store; there are some herbs that have diurectic effects. The clerk should be able to help you. If that doesn't work, ask your doc for a mild diuretic.

But first and foremost, IMO you should absolutely tell your doc. There might be something else going on like urethral spasms, a blockage, kidney problems, neurological problems (MS comes to mind), etc. It could be anything and I would not necessarily blame it on the hydo!