I thought I had a spider bite on my back but a friend of mine told me it was probable herpes zoster. Should I see a doctor even if I don´t feel pain?

Depends. It depends on the severity of the pain (if any) and truly how long it has lasted. It really depends on how you are feeling and what you feel is the best time.
Skin rash. If the rash is expanding you need to have it checked. I would not rely on a friend, zoster is preety painful, you need a proper diagnosis.