I have exactly what your describing. I have had it to a minor degree for many years on the tops of my thighs...but not too noticeable. For the last year and a half I have had my shoulders and upper arms erupt in flat white spots (they almost seem to be under the skin) whenever I'm exposed to the cold. It's gotten so bad that I could never wear a sleeveless shirt. My family doctor thought it was a fungal infection and I took oral and topical anti-fungal medications with no results. I went to a dermatologist and she said it was a circulatory problem. I also have an allergy to the cold and can break out in hives if my skin temperature drops quickly...so I have often thought it was a form of allergy, where my blood vessels contract to save heat leaving white areas. If I rub my upper arm and warm it up the spots go away temporarily.
have been taking anti-histamines lately to see if that helps...it may have slightly...I'm trying brands with different active ingredients to see if one might work better...if this is a part of an allergic response. I'm thinking about asking for a referral to an allergist.