I've had that diaper-rash problem from time to time. Not sure if there is an immediate cause (besides it just being Lupus) - I've never really identified one. Treating it like candida might help.
I just know NOT to treat it with alcohol. Did that once and never ever again. The last time I used some 10% cortisone cream I had around - that was a bit soothing and helped the pain as well.
Also I made sure the area was Extremely Dry (I used my hair dryer) before dressing in clean cotton undies. And dried it as often as needed throughout the day.
Best wishes, and I doubt it's shingles...non-typical area for that.
Lynnwood, Lupus & Sjogren's Moderator, Dx: 2002DIAGNOSING LUPUS & LUPUS RESOURCES
"Life is far too important to be taken seriously." - Oscar Wilde