The burning and irritation after applying Derobin ointment are likely due to a reaction to the medication, as it contains strong ingredients that are not suitable for sensitive areas like the inner thighs, so you should stop using it immediately. Gently clean the area with water, keep it dry, and apply a soothing moisturiser or calamine lotion to reduce irritation; avoid tight clothing and wear loose cotton underwear to prevent further friction. Do not apply any steroid or medicated creams without medical advice, as they can worsen the problem. If redness, pain, or burning persists, or if there are signs of infection like discharge or spreading rash, it is important to consult a doctor for proper treatment and a safer alternative.