I bet you aren't allergic to narcotic pain killers they just gave you too high of a dose a s you reacted like you were given heroin since you claim you are allergic means you have no tolerance. The red skin and itchiness is something heroin addicts chase.
You did the right thing - icing down or cold water can often help reduce the blood flow to the area & interrupt whatever's causing the priapism. The fact that you took care of it in well under 4 hr is great - you most likely did not experience any damage from a long-term erection. Also, it's good that you realized this was not a normal situation & were able to take care of it yourself...
This is definitely a question for a medical professional. Normally I would trust what the Urologist said, but somehow I don't feel 100% confident in that answer. It's possible he's right, but if this happened to me, I'd want to get a 2nd opinion on it. By the way, how long did the episode of priapism last? Generally we're told an erection lasting longer then 4 hrs is dangerous & needs immediate treatment. Did you go to the ER for treatment, or were you able to get it under control in under 4 hrs? I think I remember hearing that taking Pseudoephedrine can counteract an episode of priapism...