Zoster without rash is called Zoster sine herpete.

If you Google that you'll find some explanations.

One thing you can try, is when you see the doctor next:
Ask to try amitriptyline. In low doses, 10 or 25mg a day, this can help with nerve problems and healing. If it works, it points to an injury to the nerve or viral actions.

Have you tried ice on that side? Ice typically puts Herpes into
remission for some. 20 minutes tops... 2 or 3 times a day.

A diet high in arginine may encourage herpes to activate.L- Lysine helps quiet it. So taking some lysine may help. If it works, then viral cause is likely. Try 2 grams a day. You should see some effects within a week or two, if it is going to work.