Around My Neck Has Dark Patches That At Times Seem

Around my neck has dark patches that at times seem itchy. More recently I noticed if I scratch it there's a sour smell of some sort. I often thought the irritation was from a gold necklace I wear but I'm not sure. The necklace is real and I've went to see a doctor for it and was told being that I'm overweight the darkness could be early signs of diabetes. Not the case cause I've had the testing and I'm fine. I've tried prescription cremes and they seem to help a little for the moment but after I stop for about 3 months it comes right back. What could this be?