What Causes Frequent Ear Infections In A Child?

My 18 month old daughter has been suffering from an ear infection that won't clear up. She was on an antibiotic for it and it seemed to be gone, but was back in less than a week, This is her fourth ear infection since 6 months. She has had no runny nose, congestion, cough. However, she has had a few bloody snots and appears to have dark circles under her eyes. Are these symptoms of allergies? Could the ear infections be caused by allergens? Does she need tubes in her ears? Her family doctor said one more ear infection and we hadve to go to the ENT.