My 5 year old feels nauseated all the time. What could the problem be?

Lots of causes. Constant nausea could have a number of causes, including reflux, migraines, and even stress. Please take your child to your pediatrician for a full evaluation to find out the cause;once you know the cause, then the doctor can work with you on a treatment plan.
Many thinfs. It could be a number of things, lactose intolerance, an inner ear problem, acute illness, hyperglycemia. It would be best for your child to be seen by a doctor.