My Sixteen Year Old Daughter Has Been Sick Since The

My sixteen year old daughter has been sick since the beginning of November she vomits every morning, has nausea all day long, perks up a little later in the day then back sick the next morning. Doctors can't find anything wrong, labs, blood etc all normal. They have her on Zantac twice daily and anti nausea strips 3 times per day but nothing works. She's so weak and has missed so much school, sports, work. Help!