If I were your endocrinologist, I would refer you directly to a nephrologist instead of waiting for months to get a re-test. In my opinion, your creatinine rise is relatively fast and acute, and I am reasonably justified to suspect that you may have a condition called acute on chronic kidney injury. In most cases, there must be some triggers that would cause the creatinine to rise rapidly. The nephrologist is expected to identify the trigger and thus protect your kidney function.
If there is no referral ordered by your endocrinologist, I think a month is too long. A re-test after a week, to me, is acceptable.