It is possible that you may have come into contact with a new product or substance that is causing the allergic reaction on your arms. Try to remember if you have used any new hygiene products, cleaning products, perfumes, or if you have been near any new plants or animals recently. Sometimes allergic reactions can occur without a clear cause, but it's important to try and identify the trigger to prevent it from happening again in the future. In the meantime, you can take Benadryl tablets to help relieve the symptoms. If the itching persists, you can also apply over-the-counter 0.5% cortisone cream for a few days. If there are no open sores or lesions at the site of the redness, a pharmacist can recommend the appropriate products for you. If the reaction doesn't improve within the next few days, it is best to consult a healthcare professional. If you notice the redness spreading or experience other symptoms such as fever, nausea, or vomiting, seek medical attention as soon as possible.