The catch

Here's a new one: I found the perfect, most flattering dress at Zara. Fabulous, almost space-age style, yet flattering in shape and color. Under $50. Really, truly, wardrobe-revolutionizing gorgeous. One of those rare mass-market items that might have been tailor-made for the person trying it on.

The problem? From the moment I put it on, I developed a kind of splotchy rash on my neck, right where the fabric touched my skin. Then, the longer I had it on, sort of developing all around that area. It was snug in the neck, but so are turtlenecks, other shirts I own. This has never happened to me before, and isn't something I'd ever heard of happening to anybody. I suppose there are fabric allergies, but my only known allergy is to cats; how would cats make their way into a fast-fashion dress? (If you have to ask this question...) Whatever this condition was, it faded not long after I'd put on my own clothes and left the mall. (Yes, the mall - what, is a Zara on a big city street that much better?)  

So? What was this? Google is not bringing me to stories of others allergic to dresses, Zara or otherwise. This doesn't appear to be a thing. The dress, according to its label (no record of it of any kind online, as far as I can tell), is mostly polyester and part stretch material - nothing bizarre. Could it be the dye? Is there any other possibility?

I could see the temptation of deciding, princess-and-the-pea style, that this means I'm allergic to cheap clothing and simply must wear only the finest, but this item cost more than most of my existing clothes, so whatever it is, it isn't that.