Vitamin C in the common form is called ascorbic acid which tends to be more acidic whereas Ascorbates are more alkaline. The reason for this is that the vitamin c is bound to an alkaline mineral (calcium, magnesium, sodium or potassium) thus rendering it less acidic.
It could be the acidic nature of ascorbic acid or it could be that the vitamin c is acting as it should in that it is neutralizing toxins which the kidney is trying to process and expel. Are you drinking adequate amounts of water to flush the toxins out of the kidneys? Perhaps you should look at some of the herbal formulas that support kidney health. Many of them are the same such as this product from Solaray http://vitanetonline.com/description/260/vitamins/Kidney-Blend-SP-6/