What Causes Pin Prick Like Pain Under The Arm Without Rash?

My doctor says I may have shingles under my left arm, but I have no sign of a rash and the pain has been here over 2 weeks. She gave me a steroid and something called ValACYclovir that I finish up tomorrow. My question is, without a rash, why has the needle-prick like pain continued? Did we get ahead of it? Can you have shingles w/out the rash? It is very painful and I can t sleep on my side. If it is shingles, can you give me an idea of a timeline? I have shoulder surgery in two weeks and sure don t want to wear the sling around my waste if this is going to get worse.