Even if they do go away I'd still ask about it. Take a picture of it now and show it to the doctor if it clears up by then and write down her symptoms in case you forget.
I used to get some sort of rash or hives before I had my resection when I was around 16 years old. I was told that it was likely from being on high doses of steroids and was also told to stay out of the sun as that could make the hives worse. I mainly had them on both arms. They were itchy, red clusters of bumps and a little raised. I haven't had them since yet I also haven't taken a high dose of steroids since then either.
Besides sun and certain medications (antibiotics, steroids, Ibuprofen, pain medications etc) being deficient in some vitamins can also cause hives or rashes. I'd definitely let her doctor know about it. It may be a sign that she needs certain tests done to check her vitamin levels, adjust her medication or at the very least offer a better treatment so she can get faster relief if it happens again.