My hives were triggered. How did it turn into CIU?

I don't really understand how I came to today... My first Xolair shot. The past three months have been hell and my body is not my own. (I know many of you are suffering much longer. I weep for you.)

In October I celebrated my husband's birthday wearing a new dry-clean only dress that was deeply red tinted. The next day I woke up with hives where the dress had touched my skin. Never had hives before so I called teledoc who put me on a 2 week dose of antihistamines. That worked! But then the hives came back head to toe, flaming hot, and royal itchy. I saw my PCP who immediately diagnosed CIU and referred me to an allergist.

Eventually I'm on a mix of antihistamines and hydroxyzine that calms the hives, but I still have more than 50 a day and the itch persists. That is why I got Xolair today

My question...I don't understand the diagnosis. CIU is supposed to have no known cause. I know the cause. That damn dress!!! Never again. Do not wear anything that isn't washed first.