It could be an allergic reaction to the things that are replacing gluten, such as soy and nuts. I have an allergy to both soy and tree nuts, and while it doesn't cause a rash, it does cause a pretty bad reaction depending on how close to the pure form of the allergens are. For example. I don't get a noticeable reaction to soy sauce, but tofu or soy beans make my throat all itchy and that's all I know because I've been careful not to have much of it. It's likely that a high amount of allergens could cause a life-threatening reaction, but I'll hopefully never know the extent of my food allergies. I also have to minimize or cut out gluten, but I have to check the ingredients whenever I get something that's gluten free because soy and tree nuts are the most common replacements for gluten. Nowadays, I stick with oat-based fake dairy and gluten free products, but I have heard that some of them can irritate celiac. I recommend getting extensive allergy testing done, if you can.
Also, it's possible that the rash is from the endo. Skin problems can be a symptom of the hormonal issues. I had eczema when I was a kid, but it eventually went away. In the past few years it has come back occasionally. I also have a skin condition called HS, but that doesn't sound like it's what you have. I think it's either the hormones or allergies, and I think the best way to find out is to do allergy tests. You're not crazy, you just have a very complex illness.

I have a rash that has come and gone on my chest and neck. I have medicated cream that worked for the worst visible rash. Aloe works as well caught early and consistent. Mine seems increased with stress. Now the endometriosis is removed I think it help.

I have had itchy itchy skin for 6 months. It flares up every day. The symptoms are those of chronic urticaria (chronic hives) and dermatographism (skin writing)
Clothing, taking off socks, tags, blankets (materials) almost anything makes it break out. It drives me crazy. The only thing that helps for a few days is Zyrtec.

On my neck, but I am not ruling out eczema because the weather is dry. It got bad a few months ago and I covered my neck at work.

Are you taking oxy for pain? My mom had that issue when she took it. It looked like hives and a bumpy rash everywhere.