Gosh, I am so sorry.  Your story resonates with me because a few months ago I was pretty much in a similar state to your mum.  My mum died quite suddenly in January and looking back on it I can see now that I really lost the plot.  I don't really remember how I spent my days.  A lot of sleeping.  A lot of crying.  I dropped my son to school at 08:30 and went back to bed immediately afterwards and only got up to go and collect my son later from school.  I just had no energy.  I was exhausted.  I had very few meals.  I forgot to eat.  People might say this is depression but as I understand it depression does not simply go away by itself.  I would say I was grieving.  I needed to look after and cook for my son in the evenings and at weekends and of course I did and I was sociable with him so I think that saved me from sliding completely into oblivion 24/7.  But when he was not at home I was a total recluse.  I did not open my front door or answer my phone or meet friends or go out.  After several months of this weird behaviour I became very worried that I was mentally ill or something.  My son was away on a school trip for a week so I booked myself into a yoga retreat while he was gone.  I was afraid that if I stayed at home I would be in bed the whole week.  I promised myself that I was going to be normal when I got back home.  It was a huge effort but I managed to do 4 or 5 yoga classes a day and a 6 mile walk every evening at this yoga retreat.   I was on the go at 06:00 each morning.  And the strict vegan diet of no animal products, no bread, no sugar, no coffee, no alcohol, etc was nutritionally exactly what I needed.    I think I arrived there malnourished.  I went home bursting with energy and I have been absolutely fine ever since and my life is back on track.  I have not had a single day of relapse into the narcoleptic state that I was in before.  

I think your mum is grieving and she will be back to herself just as soon as she can.  Grief affects different people in different ways.  It hit me like a freight truck.  I was absolutely knocked down.   Some people throw themselves into their work.  Some people go to pieces.  I cannot explain my own behaviour.  I am ashamed of it, really.   But my mum was a huge influence in my life, she was mother and father to me and my best friend.  I had no time to adjust to the situation.  One minute she was 100% well and then suddenly diagnosed with cancer and dead 10 weeks later.  Total shock.   I remember also that I could not relate to my brother who seemed to be able to carry on as if nothing had happened, he did not even miss a day off work and was far too chirpy and upbeat even at our mother's funeral.  I was devastated by that.  People all around me kept telling me over and over that I should pull myself together and move on.  Even after only a fortnight.  It seemed indecent to me.  Highly insensitive.  It hurt too much to be continually subjected to that so I withdrew completely from family and friends.

I suppose I am defending your mum's behaviour.  It is still very recently that her mum has died.  So I think she is allowed to go to pieces.  The thing I worry about is her misuse of sleeping pills and not taking her regular medication.  Her GP should know about this.  

Your mum will be back to herself.  She just needs time.  She is not choosing this behaviour.  Nobody would.  But if she has already a history of depression and now is dealing with grief and is not taking her regular meds then I think her GP must know about this straight away as she may need some help getting back on her feet.

I do hope your mum will soon be back to the mum you are missing.  She is lucky to have you worrying about her and asking for help and advice.   Your mum's own friends who have also lost their mums might come forward now and visit your mum and talk to her about this shared experience since your mum feels that nobody in the family understands what she is going through.  It always helps to talk to people who have the same shared experience.  The sleeping is a problem.  The more sleeping you do, the less energy you have.   It's a vicious cycle.  Somehow she needs to get out for fresh air and a bit of exercise. It makes a huge difference.  I feel for you because I was exactly lying in a heap like that myself and I just could not muster the energy to do anything.  It was like having M.E. or something.   You must be so worried.   I really think her GP needs to know the full story on this.  

Best wishes, Danielle