Hello,
This can be due to contact dermatitis, allergies, eczema ( presenting as red, flaky, itchy rash) or notalgia paraesthetica. Notalgia parasthetica also presents as itchy, dark pigmented patch
diseases like scleroderma and SLE have to be ruled out. For that you need to consult your dermatologist.
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 doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.