Overdosing on vitamin C also causes diarrhea.
If the dose you are taking causes burning urine or issues behind, then cut back your dose by about 20% and see if that solves the runs. I have taken as much as 24 grams over a 24 hour period and not had that problem. In an isolated wilderness situation I once gave 50 grams of C to a woman who had been stung by 3 yellow jackets in the neck but had left her bee allergy kit at home. This was over an 8 hour period. I had given her 10 grams crushed and mixed with water immediately, and in 20 minutes she was no longer gasping for breath, her eyes were now open, and the swelling all over her body was subsiding. Then 5 grams an hour until she told me to go back to my tent and sleep, she could estimate her dose based on how she felt. She never had any diarrhea problems and she walked out 3 miles the next day with friends.