Blotchy rash on thighs, not raised or itchy

Today my adult daughter suddenly had a blotchy, red pattern on her thighs that did not cause itching.  The rash was not bumps, and not raised - just weird looking, with wide, red lines swirling randomly over an area of several inches on each leg.  She had not been exposed to the sun, and in fact was wearing long pants which she's worn before.  It remained for a couple hours, after which time she left so I don't know how long it ended up lasting.  Maybe it's stil there.  It doesn't exactly sound like the symptoms for lupus, which can sometimes cause a leg rash; however, if she does spend much time in the sun, she gets little blisters across her chest, and it sounds like that could be related to lupus.  As a mom, I'm probably jumping to wild conclusions, but it seems like there must be a reason for someone's system to manifest a rash.  It's not ringworm because that has definite circles.  I would appreciate any comments -thank you!