It sounds like you are experiencing an allergic reaction, likely contact dermatitis from the hair treatment chemicals. Since you have already taken fexofenadine (Allegra), this should help reduce the itching. You can gently wash your scalp and hair with lukewarm water and a mild, fragrance-free shampoo to remove any remaining chemical residue. Applying a cool, damp compress or a mild, over-the-counter hydrocortisone cream to the affected skin can also provide relief. Please avoid scratching to prevent infection. If the rash continues to spread, or if you develop swelling in your face, lips, or have any difficulty breathing, please seek immediate medical attention.