Dry Skin (Xerosis): Given the patient's complaint of abdominal itching without any other systemic symptoms, and the fact that over-the-counter allergy medication did not help, dry skin is a plausible cause. The skin's sensitivity to clothing could be exacerbated by dryness, especially if the patient has not changed their detergent or soap, which might still be irritating despite no recent changes.

Eczema (Atopic Dermatitis): This condition can cause itching and skin sensitivity. Although the patient denies changing detergents or soaps, an underlying eczema could be exacerbated by environmental factors or could be a new onset.
Contact Dermatitis: Despite the patient's denial of changing detergents or soaps, contact dermatitis remains a possibility, especially if there has been exposure to new fabrics, dyes, or other substances that could cause a reaction.
Allergic Reaction: The patient has already tried over-the-counter allergy medication without relief, but an allergic reaction to something in their environment or diet cannot be ruled out without further investigation.

Lymphoma: Although rare, lymphoma can cause itching without a rash (pruritus sine materia). It's crucial to consider this diagnosis, especially if the itching is severe and persistent, despite the lack of other symptoms.
Hepatic Disease: Certain liver diseases, such as primary biliary cirrhosis, can cause itching due to the buildup of bile salts in the skin. This diagnosis is critical to consider because of its potential impact on the patient's health.
Thyroid Disease: Both hyperthyroidism and hypothyroidism can cause skin changes and itching. Given the potential systemic implications, it's essential to rule out thyroid disease.

Polycythemia Vera: A myeloproliferative disorder that can cause itching, particularly after warm baths or showers, due to the release of histamine from basophils.
Neurological Causes: Certain neurological conditions, such as multiple sclerosis or neuropathy, can cause itching sensations without a rash. These are less common but should be considered if other diagnoses are ruled out.
Parasitic Infestations: Although less likely, parasitic infestations like scabies or pediculosis could cause itching, especially if the patient has been in close contact with someone infected.