Look into wheat allergy! I have the same exact thing you do. It so itchy and annoying. I finally fugured out that I am allergic to wheat. Read up on it. I am telling you thats what it is!
The rash that you are having and the joint pain can be related as in psoriasis and psoriatic arthritis. The rash can be due to psoriasis which causes skin cells to grow too quickly, resulting in thick, white, silvery, or red patches of skin. It is thought that psoriasis occurs when the immune system overreacts, causing inflammation and flaking of skin.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
My wife has the same symptoms, did you find anything out.