We had an ice strom in Austin, TX a couple of weeks ago and since then the pressure in our shower has decreased considerably. We don't see decreased pressure anywhere else and we don't see leaks anywhere in the house. What could be the cause of this?

Here is a key question. Was the supply line to the shower on an outside wall facing into the weather? If so, it possibly froze and the dislodged son crud in the pipes. First remove, inspect and clean the shower head. If that doesn't help shut off you water main and remove the valve stem under that shower handle.