Breathing struggle Life and death matter

My mother who has a cardiac condition is now suffering from breathing attacks on a more and more regular basis. She is now in hospital for a while, they have check everything, heart, lunghs , arteries.... and nothing seems to be the problem.
The attacks come mainly at night time., she suddenly feels some difficulties to find air supply and in a matter of minutes she gets no more air at all. She does not have fever, The emergency teams gives her oxygen and we do have supply at home as well but it seems that it is not the real issue as tests shows that she has enough oxygen in her blood.
Has anyone come accross this condition? What can do such effect on a body?
We are now thinking of a very strong allergic reaction to something.... what we don't know.
Please help, this is so worrying.
Thank you for your answers