@fionac What you describe are the classic symptoms of gastroparesis (GP), loss of appetite, feeling full very quickly when eating, nausea and unintentional weight loss and can also include vomiting, bloating and abdominal pain or discomfort.
These symptoms can be mild or severe and can fluctuate from day to day. An endoscopy will sometimes see excess food still in the stomach as something of a guide for a GP diagnosis but if you hadn't eaten much in the day/s prior to the procedure or your just having a good motility day, the stomach can look o.k. Gastritis can be a common finding too but you can severe GP without any sign of that.
A Gastric Emptying Study is the best test to check for GP but because of the fluctuation in symptoms (on on hourly basis, let alone a daily basis), it can often return a normal or low normal result. A motility agent such as domperidone is the go to drug (at least in Australia) that a G.P. will prescribe first to see if it alleviates symptoms but there's a fair percentage of those with a GP diagnosis that don't respond to this, or other motility agents.
Hypoperfusion of blood flow to the bowel is one mechanism that can cause GP, especially in those with low blood pressure with other autonomic dysfunction too, some common symptoms that will be easily recognize here on PR. Gabapentin is a commonly prescribed drug for neuropathic pain in those with GP.