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This could have something to do with the amount of protein you are eating. Even though you are weight training, in general people with some kidney disease don't tolerate large amounts of protein. It might actually cause more damage.
Depending on how long you have been doing this weight training program---it may also alter the way your body is responding the medication for diabetes. Also, as people age, any chronic diseases you have can change along with the normal body changes of aging. It is not unusual for diabetics to have to alter their medication, diet and exercise regimen to maintain control as time goes on.
The best thing to do is to discuss this with your doctor that follows you for diabetes. If your "estimaged" GFR is correct, you have entered into stage 3 kidney disease which means you need a tighter control of blood sugar, blood pressure and possibly a special diet as well. You may need to consult with a dietician; and your doctor should refer you to a nephrologist (if you are not already seeing one).