What Causes Foul Discharge From Ear That Forms Sticky Crust On Drying Up?

My 5 year old daughter has tubes in her ears. she had persistent ear infections as a baby. several years have passed since they were put in. for the last week or so she has had a smelly yellow discharge coming out of her left ear. it dries and forms a sticky crust. do i need to take her in for antibiotics? i figured it would have gone away by now?