Sorry to hear you are going through all that.  

I think it is important to remember though that the options are not only (or even mostly) it being all in your head or being something life-threatening. Did the doctors say anything about the ovarian cyst? Is it possible that is causing some of your symptoms? Things like IBS or lactose intolerance can also cause some of those symptoms and IBS is mostly a diagnosis of elimination, as far as I know (though I am not a medical profession and am going mostly on what I read here) so you wouldn't expect to find that in tests.

Most illness are not life-threatening and what you describe sounds like it could be many, many things, most of which not going to put your life at risk. And I wouldn't worry too much about the weight loss in your case (well, to be honest, I probably would, because it's easy for the rest of us to say there are plenty of minor or treatable causes but when it's oneself or somebody you love, it's hard not to think "but what if it is the worst?") because you are having diarrhea and nausea and are probably eating less than usual. When I had the winter vomiting bug, I lost nearly a stone in a week. You've been having stomach symptoms for months. It would be surprising if you hadn't lost weight.

It does sound like stress is possibly a contributory factor in your case. This does not mean it's all in your head. Stress can bring on physical symptoms and heck, you've had a stressful year, we've all been through a pandemic, your father and sister have been ill and you've had an ovarian cyst found (I know your symptoms started before that, but it was bound to add to your stress). Having a stress reaction and being run-down after dealing with that isn't imagining stuff. 

Sorry to hear you are going through all that.  

I think it is important to remember though that the options are not only (or even mostly) it being all in your head or being something life-threatening. Did the doctors say anything about the ovarian cyst? Is it possible that is causing some of your symptoms? Things like IBS or lactose intolerance can also cause some of those symptoms and IBS is mostly a diagnosis of elimination, as far as I know (though I am not a medical profession and am going mostly on what I read here) so you wouldn't expect to find that in tests.

Most illness are not life-threatening and what you describe sounds like it could be many, many things, most of which not going to put your life at risk. And I wouldn't worry too much about the weight loss in your case (well, to be honest, I probably would, because it's easy for the rest of us to say there are plenty of minor or treatable causes but when it's oneself or somebody you love, it's hard not to think "but what if it is the worst?") because you are having diarrhea and nausea and are probably eating less than usual. When I had the winter vomiting bug, I lost nearly a stone in a week. You've been having stomach symptoms for months. It would be surprising if you hadn't lost weight.

It does sound like stress is possibly a contributory factor in your case. This does not mean it's all in your head. Stress can bring on physical symptoms and heck, you've had a stressful year, we've all been through a pandemic, your father and sister have been ill and you've had an ovarian cyst found (I know your symptoms started before that, but it was bound to add to your stress). Having a stress reaction and being run-down after dealing with that isn't imagining stuff.