I am so sorry for your reaction! I'm not even sure Benadryl would've helped if you took it beforehand. Are you allergic to shellfish and don't know it, by any chance?
Did they admit you? What did they do for you? Stare?!!
You, as a patient, have the RIGHT to say "NO" to any test or substance! It is NOT required. They "like" to have contrast but I usually say NO!!!
Have you spoken to your primary doctor? That might be a very good idea. At the very least, they should check your kidney and liver functions. I don't know what other drugs you're on but at least make sure that the iodine is out of your body. There are chelating supplements you can take (i.e., Zeolite) but you need to speak to your doctor before doing anything. You don't want to make things worse.
I assume you let your neuro know about this too?
Do you have a medical alert card and/or bracelet? I think that's very important. What if you are unconscious and no one can speak for you? You don't want this sort of thing happening again.
I hope you'll get better. What a scary thing to have happen. Take care.