Most hospitals either have regular meetings or prepare internal reports -- often weekly --in which they review any deaths that have occurred in the hospital during that period. This is
often called a mortality or morbidity review/report. Particularly given the circumstances surrounding your mother's death, the hospital almost certainly conducted some kind of internal review. The problem will be for you to get access to any records of it. The first place to go for help would be your mother's primary care physician or the physician who was in charge of her care at the hospital. Let the doctor know of your concerns and ask if he or she got a good explanation from the hospital, and whether there is any documentation about what happened. Doctors are very protective of each other, however, and there are also medical privacy laws that restrict information, so it may not be easy to get a lot of information from your mother's doctor.
Another place to try to get information about the hospital's review of your mother's death is the ombudsman for the hospital. The ombudsman is a patient advocate who helps patients and their families sort through problems with the hospital. You can contact the hospital ombudsman simply by phoning the hospital's general number and asking to be connected to the ombudsman.
There is also the official death certificate for your mother. This certificate will list cause of death, but most death certificates only give a very brief and simple cause, without going into details. Still, there may be something in the death certificate that gives you a hint about what the true cause was of your mother's death. The hospital or the hospital ombudsman can help you get a copy.