Where on the arm exactly? What are the symptoms?
I would see a medical provider to verify your diagnosis.
Have you been tested to know you even have herpes? Herpes just doesn't randomly spread around the body, in fact autoinnoculation is uncommon. You would need to get a swab/culture of any lesions to know it's herpes going on - you can't self diagnose.
Both you and your partner should get blood tests to know your herpes status. Request an IgG type specific blood test at this point - and of course be seen within 48 hours of symptoms for a swab/culture.