You can be certain that a rash on the arm is not due to HSV-2 and almost certainly not HSV-1. "
PainfulPainful menstrual periods blister clusters" is a great description of herpes, however, and herpes zoster (shingles) sounds like a possibility. Although most common in older people, shingles is not rare in younger, sexually active persons (my son had it at age 13).
Shingles is a recurrence of childhood chickenpox in localized form. It does not travel to other parts of the body. It cannot be transmitted to people who had chickenpox as children or who have been vaccinated against chickenpox--which includes almost everybody. So if that's what your partner has, you have no worries about catching it in your genital area or anywhere else.
Your partner should see a health care provider immediately to see if he has herpes zoster. If so, his provider might prescribe an anti-herpes drug like valacyclovir or famciclovir. Some cases of shingles are followed by severe pain due to post-herpetic neuralgia, which can continue for months or years; the risk is reduced by treatment, but only if it is started early. So don't delay.
But don't worry about genital infection or transmission to you. Good luck--
HHH, MD