What Causes A Rash In The Back After Taking Vicodin For Back Pain?

Hello,Been having back pain for an unreason started this week it's very painful located in the same place been on Vicodin for it and now I noticed a rash in that same painful area and I'm concern it may be shingles. How does shingles get diagnosed? Is there a test or it by looking at the rash, my rash looks like little blisters.