What could have caused these patches to develop on my arms?

Thanks for the question. If you have a diagnosis of psoriasis, there is a possibility that certain treatments can aggravate it. With psoriasis, damage or injury to the skin can cause psoriasis to develop at the site of the injury. This is know as the Koebner phenomenon. Therefore, you do have to take care that the treatment is gentle and does not cause any trauma or damage to the skin. Likewise, for eczema, the skin is sensitive and more prone to external insults, so you should avoid treatment if the facial skin is still red or irritated as this can be more easily aggravate by treatment.

If there’s a definite temporal association between using the new product and the appearance of the itchy red rash, then yes it’s likely that it’s some form of dermatitis related to the product. Dermatitis is also more likely if you are not getting the rash anywhere else on your body, and also have never had the rash before, nor any personal or family history of eczema/psoriasis. The fact that the red patch comes and goes however warrants further investigation by your doctor – there may be something else in this story.