One of the challenges of adulthood is the evolution of the parent-child relationship.
Consider that your mother has the same right to privacy as any other individual whose adult child is not a nurse. People choose to share or not share their medical issues with family members according to their relationship patterns and histories, their personal perceptions of privacy and appropriateness, and their personal feelings about illness etc. ... not necessarily based on whether or not a family member is "qualified" to provide some insight.
Having said that, I agree with the poster above who suggested that you can communicate your concerns with your mother's provider. At the same time, perhaps a softer approach with your mother might be helpful - something along the lines of, "Mom I know that maybe you're going through a rough time and I just wanted you to know that I love you and I'm here for you." Take the fact that you are a nurse out of the equation right now, and just support her, even from a (virtual or emotional) "distance", if that is what your mom wishes. There may be a long-standing mental health issue of which you were not aware. Or there may have more recently been something going on in your mother's life that she may not wish to share with you right now, or ever.
Wishing you & your mother well.