17 years old female with morning nausea/vomiting, occassional diarhhea (not pregnant)

My 17 year old child (female) is having digestive issues, morning nausea (she is not pregnant), and somethings vomiting, occassional diarrhea and sometimes constipation.  She had the same issues last year, we took her to a Gastro and he ordered an abdominal unltrasound (came back clear except for constipation) they imaged her upper and lower intestines and her kidneys as she was complaining of occassional back pain.  They found nothing, than ordered an endoscopy, came back clear, they did a biopsy of the stomach lining benign and no signs of celiac.  The ultrasound found constipation, put her on a course of Miralax and she was fine in about 2 weeks.  Now she is having similar symptoms but not constipated that we can tell.  I want to take her back to the Gastro, should I request an upper and lower GI with contrast? What other things should the check are there blood test to check liver and gallbladder?  Just want an idea of what to ask Doctor about before we go. Thanks, Concerned Mom.