How tight are you wearing the watch? This sounds to me either like a reaction to a metal used in the band/case back or your skin reacting to extended pressure and a lack of breathability under the strap. If it's either of those simple hydro-cortisone (or similar) steroid creme and leaving the watch off for a few days should clear it right up. If it's the breathablity issue than just take your watch off at night and that should prevent it from recurring. If not get a referral to a dermatologist and consult an expert
