Shingles (Herpes Zoster): This condition is characterized by a rash that typically affects one side of the body, following a dermatomal distribution. It is often accompanied by pain, but in some cases, the pain may be minimal or absent, especially in the early stages.

Contact Dermatitis: An allergic or irritant reaction to something that has come into contact with the skin, causing a rash on one side of the body. This could be due to plants, metals, or other substances.
Psoriasis: Although psoriasis can affect any part of the body, it sometimes presents with a rash on one side, especially if it's localized. The lack of pain is consistent with some forms of psoriasis.
Eczema (Atopic Dermatitis): Can cause a rash that might be localized to one side of the body, depending on the area of skin affected. Itching is more common than pain.

Lymphoma Cutis: A rare form of lymphoma that presents in the skin. While it's unlikely, missing this diagnosis could have severe consequences, making it crucial to consider, especially if the rash is persistent and unexplained.
Metastatic Cancer: In rare cases, cancers can metastasize to the skin, presenting as a rash. This would be an unusual presentation but is critical not to miss due to the implications for treatment and prognosis.

Linear IgA Disease: An autoimmune condition that can cause a blistering rash, which might appear on one side of the body. It's rare and might not cause significant pain.
Dermatomal Vesicular Rash due to Enterovirus: Certain viral infections can cause rashes that follow dermatomal distributions, similar to shingles but without the pain, and are much less common.
Phytocontact Dermatitis: A rare condition caused by an allergic reaction to plants, which can cause a rash on one side of the body if the exposure was localized.