What Causes Red Bumps Under Chin While In Treatment For Pneumonia?

My 11 month old has been treated for pneumonia for the last 8 weeks with injections, IV s, breathing treatments, oral and inhaled steroids, & 2 types of oral antibiotics. About 6 weeks into those treatments she developed itchy red bumps under her chin that over a few days spread to her trunk and extremities. I have taken her to see the doctor and they said it is likely caused by a virus and to give her diphenhydramine PRN. I give it to her q 6 along with a hydrocortisone ointment to keep her from constantly whining & scratching herself. She has also had a stuffy nose and intermittent diarrhea/upset stomach the entire time she has had the rash. I feel like this may be more than a virus.