Whats causing this nerve pain in my back?

ever since the cold weather came around I've been having nerve pains on my back. there are sort of pimples or spots where it's painful. usually it's just small pin pricks but sometimes i get big painful flare-ups, usually where my bra strap is. i did have shingles Nov. 2009 and the symptoms are similar to postherpetic neuralgia, except it's on both sides of my back. sensitive to cold. small muscle pains often precede large flare-ups. anyone know what it could be?