What Could Cause A 3-year-old Child To Get Impacted When

What could cause a 3-year-old child to get impacted when he eats a healthy diet with lots of fruits/vegetables and drinks water. He has never even been constipated. His stool is chalky colored. He is having somewhat watery stools and is vomiting up what the ER doctor said was feces. The triage doctor did an x-ray and found no obstruction and said that the bowels were full. He continues to have bowel movements without taking Miralax that the doctor prescribed. He also prescribed Zophran for nausea. The child is not nauseous. He just throws up sporadically and screams saying his lower stomach hurts. Once he throws up, he seems fine...for awhile. Should we be concerned?