Definitely keep a notebook.

Contrast can cause a reaction, and you're going to want to know that for future reference so you can avoid it if necessary. It is also possible to have stress hives (I get these at times), though I think those are more likely in folks who tend to already have funky immune systems. I haven't researched it. But I have certain spots that break out specifically when I get stressed (separate from shingles, which I have also had) and I can tell them from allergic reactions because of the location.