That could exactly be what it is, depression. If they can't find anything physically wrong with her to explain away the symptoms, depression can and does cause those exact set of symptoms. Fatigue, loss of appetite, nausea, unable to motivate yourself to even get out of bed etc etc.

She's been through a helluva lot and depression can come once she's kinda left to her own devices once things are all done. PTSD is another disorder that can hit people afterwards too.

Proper depression is underplayed nowadays because a lot of people use it interchangeably because they happen to be down for a day or two.  Proper depression, clinical depression is nothing like that.  You simply can't lift yourself out of it. If you suspect she is indeed suffering from depression, maybe talk to her to see if she needs help. Like proper help.

Hi Nicola FB

Sorry to hear your mum's poorly I'm not sure what a Plasymacytoma is so I don't know what to say ...I had 5wks of Radiotherapy in June ...Radiotherapy effects your appetite it did me ...I think having Cancer leaves you with a fear ...a fear of it coming back ...I have been anxious/depressed with it all and I think your mum has depression ...plus she has been in and out of Hospital which will add to it ...I hope things get better for you all soon ..godbless. 

Thank you for this.  I think it's the fact she is not eating at all that's the key problem.  She then has no energy to do anything. But I will definitely suggest she seeks some counselling or other form of professional help.