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.
Where on the arm exactly? What are the symptoms?
I would see a medical provider to verify your diagnosis.