How to differentiate between GP and non GI nausea?

I got a concussion in June and I’ve been all sorts of fucked up since. I’ve lost ~15 pounds (from 130) and I feel sick and horrible all the time. I went to GI, they did an endoscopy that came back with only benign inflammation and a GES that came back with 40% retention after 4 hours. However, I honestly don’t know if GP is really the thing causing my issues, rather just a complicating factor. A lot of my nausea seems more related to POTS and heat dysregulation but I have no idea how to tell. The GI I saw just wrote “eat a low fiber and fat diet and return in 2-4 months” but I don’t know what I’m going to do until then because I went to GI mostly so I could get more antiemetic options than just Zofran.