Sounds like Eczema, but without seeing it I can't say for sure what it is. It is possible to get this EVEN if you have never had it before.
Is something constantly coming in contact with your neck and shoulders and rubbing, i.e. hair with products in it, clothing, etc.? Have you changed laundry detergents or using new body lotions or body wash?
Keep using the cortisone and keep the area open to air when you can. Sunlight/light sometimes helps; not OVEREXPOSURE to sunlight. Do NOT USE any other lotions or cream on it. Wash the area with plain water and pat dry.
I would recommend consulting your physician or a Dermatologist because you probably need a Rx cortisone and possibility something else prescribed.
First do you where jewelery around you neck? If so it sounds like exezma. the combination of mainly fake jewelery and sweat causes a rash but also it doesnt have to be fake jewelery either just the combo and rubbing of sweat and jewelery can cause this. I would try to stop whearing anything around your neck and get some cortisone 10 cream from your local CVS and it should go away but you must not where a necklace until it heals then again you might not even want to when it heals if you dont want the rash to come back... hope this helps
Metal allergy does sound like a possibility. I know of people who can only wear pure gold. Not white gold, which can have other metal in it, but only pure gold. This means the yellow kind.
If you can't afford the real metal, you can try semi-precious stones. But, make them yourself, so you can use the proper metal findings for the ends or make them long enough that they don't need clasps. This would avoid the whole metal allergen thing all together.