Blood pressure numbers are written as 140/90. We say "140 over 90." The top number is called systolic. The bottom is diastolic.
A diastolic pressure of 40 or 50 can be normal for a young person who is healthy and feels fine. As we move from our twenties toward middle age, diastolic blood pressure tends to rise. So, a diastolic reading of 50 or less in someone over 40 years of age would be unusual. It could be normal for that person, but I would be thinking about heart or vascular problems.
Many people develop hardening of the arteries (atherosclerosis) as they get older. Atherosclerosis causes arteries to become stiff. When the heart pumps blood into stiff arteries, pressure inside the arteries rises quickly to higher than normal levels. The pressure then falls to a lower than usual level at the end of a heart beat. This often translates to a high systolic reading and a slightly low diastolic reading. Sometimes that diastolic reading will be as low as yours.
Since you have diabetic kidney disease, I suspect you take medication to both lower your blood pressure and help slow further decline in kidney function. If your top number is above 130, you may have to accept the low diastolic reading. However, if you get lightheaded when you stand or have coronary artery disease, talk to your doctor. You may be able to decrease your dose of blood pressure medication.
Blood flow in the coronary arteries occurs during diastole. That's the heart's relaxation phase after each contraction. If you have some blockage in your coronary arteries, then a low diastolic pressure may impair the ability of oxygen-rich blood to reach all parts of the heart.