Based on your description, it seems like your doctor has provided you with incorrect information regarding HIPAA violations. There are exceptions within HIPAA designed for just this type of situation.
If your mother named you her designated legal representative or the executor of her estate, you have the right to access her records. Depending on a variety of factors, including the state where your mother resided while alive, the fact that you are a member of her immediate family should give you the right to her records as part of default relationship hierarchy. If you provided financial or supportive care to your mother while she was alive or had access to her medical records previously via a medical power of attorney or HIPAA authorization document, you should also be allowed access.
Of course, there are exceptions. If your mother designated someone else as her legal representative or executor, then you would need that person to retrieve the records for you. Otherwise, you should only need to provide the doctor with legal proof of your status, such as a birth certificate or notarized power of attorney document, and the death certificate. In some states, you might also need to go to court to gain a special form of proof not outlined here.
At this point, you should ask the doctor for a list of acceptable documentation. If the doctor isn't helpful, contact a patient advocate at the hospital system that your doctor has a relationship with in that region. If all else fails, speak with an attorney who specializes in healthcare and HIPAA rules or estate planning. Keep in mind that states have their own rules as well and that the doctor's office might be telling you that they have HIPAA violation concerns when they're actually dealing with a state rule that blocks access.