Hello mkirby19 ,  My heart goes out to you as my daughter went through something similar and finding no answers is torture.  My daughter started complaining of stomach pain, feeling nausea and vomiting off and on from the age of 11.
  For over 3 years we had every test you can think of and no real answers. When no answers were found they blamed it on her nerves, depression, etc and she had to be home schooled. Then a doctor decided to check her hormones and found that she was producing to much Prolactin  and he ordered a MRI of her brain which turned out to be a pituitary tumor. This type of tumor I was told will not show on a cat scan.  Because all of her complaints had to do with the stomach we never thought to check anything else. She did not exhibit any of the normal symptoms people show with this type of brain tumor. I was later told that nausea and or vomiting that continues for long periods of time ( even if it's not everyday) should always be checked out for tumors.  My thoughts are with you and your daughter, nothing in this world is worse then having a sick child and not getting any answers.