Hello, Welcome to healthcare magic. From your history and description, the possible differential diagnoses I can think of are pressure urticaria and contact dermatitis. Pressure urticaria presents with raised, itchy rashes (hives) at the sites of pressure (usually the bra straps and waistline where a belt is tied, back of thighs if you have sitting for a prolonged period, etc). It lasts for a few hours and disappears. The exact cause is unknown and it can persist for years. You could benefit from taking an antihistamine like levocetrizine or loratidine. Contact dermatitis could occur due to allergy to the elastic material in the bra strap and undergarments. It presents as itchy, diffuse rashes which generally persist. Please consult a good dermatologist and try to confirm the diagnosis so that you can treat appropriately. Hope this helps you. Take care.