Need advice about confusing symptoms: nausea followed by headache

[RuthieJ18]

Hello, I could really use some advice about a problem I have had for 3-4 years with episodes of nausea and vomiting, often followed by a terrible headache as well as other symptoms.

Initially, I would be woken at random several hours into the night with sudden onset of vomiting undigested food. I would be violently ill with sweats, shaking, and ears ringing as if I was going to pass out. This would happen every 2-3 months or so. I also had some gnawing stomach pain.  I saw my doctor who tested me for h.pylori (negative) and started me on a PPI (lansoprazole), which did seem to help for a while; although I would have episodes of nausea, I did not actually vomit as often.

After a while the episodes of nausea became more frequent and I noticed I often would develop a terrible headache after being nauseated. My doctor suggested the nausea was an aura followed by a migraine, but I am skeptical as the headache does not occur until up to 12 hours after the nausea begins and I do not have light sensitivity with it.

I have noticed I have uncontrollable yawning when nauseated and a feeling of fullness in my abdomen, even hours after eating. It does seem to feel better if I have just a little something in my stomach at all times. Although the nausea is now as frequent as 2-3 times a week I haven't lost weight because I still eat pretty often. I have tried Carafate, Zofran, and Phenergan but would rather find the cause than treat the symptoms.

So far have had gallbladder ultrasound (normal), HIDA scan (low gallbladder function at 37%), brain MRI (normal), and endoscopy, which showed some evidence of mild gastritis and a possible GIST tumor. My doctor doesn't seem concerned about the GIST, stating it can't be causing my symptoms but I do feel that it needs to be explored further.  

What would be further testing that I request? This is really impacting my life and work and I am desperate to feel well again.