My son has been on Amoxicillan for a sinus infection for the past 8 days. 3 days ago, he developed a red rash in his anus area. We have been treating it with diaper cream, but tonight, we noticed the red rash has become hard, white bumps. They feel very rough. He is complaining his butt hurts quite a bit now. As of today, he is off the amoxicillan, since it was not effective. Could this be an allergic reaction?