I can't take 10 g of Vit C either. It's either too much for you, or too much at once if that's how you took it. Just reduce until you don't get diarrhea any more.
I suspect both that you're not getting enough Iodine and that you're not taking enough magnesium. It can be tricky to get enough magnesium, since it too can give you diarrhea. I use both magnesium citrate and magnesium malate, which allow me to take more magnesium than I could get with magnesium oxide.
As an experiment, put your Iodine in orange juice to see if you tolerate it better. I believe that would give you Iodide only, on the chance that the elemental Iodine is a problem.