Strange Sunburn Like Itchy Red Rash Around Neck? Any Idea's?

I began to get a strange sunburn like rash around my neck only a few months ago (it has now gotten worse). It feels very systemic. It is not a typical rash. It is not raised. There are no bumps or scaliness. I posted in the thyroid forum because I am having thyroid issues and wondered if it could be related but my endocrinologist doesn't seem to think so (of course doctors are notorious for missing things that can be obvious). It's only around my neck and it's making me miserable. I have not eaten anything new. I have not started wearing any new perfumes or using any different detergents. No new foods or drugs...nothing. No jewelry either. I've tried different antihistamines and a bezillion ointments and creams (Sarna Ultra is the only thing that even gives me any semblance of relief).

The only other thing that I have done differently from my normal routine is that I began to use Differin Gel 3% on my face only which is why I never thought of it as being the cause of this strange neck rash, but a friend at work said that I should look into this so here I am. Is this possible? I have heard that Differin can be a bit systemic for some. I never used this on my neck so I am wondering if anyone else has had any strange rashes like this from Differin in places that it was not applied (like in an area that was near where it was applied)? If anyone has any ideas in general that would be great because this rash is scaring me a bit now. I'm a breast cancer survivor so I don't know if this strange rash is signaling that something is wrong systemically or what here. My GP is at a loss as he has never seen a rash like this pop out of nowhere on a persons neck only that has a sort of necklace pattern even though I do not wear jewelry nor have I ever had a rash like this. It's flat, angry and looks like a sunburn though it itches like crazy!

Anyone have any idea's? I would greatly appreciate any help someone might give here. Thank you. :)

[GravesGin]

I also have a strange type of Graves disease and am a breast cancer survivor. I developed this red and itchy rash around the base of my neck about a year ago. It is burning sometimes. My dermatologist is stumped. It went away once but recently came back. Tonight it is bothering me so much I am frantically looking on the internet to see if I can find out anything. I would like to talk to you.