I know that the Slo Fe is a time release iron pill. Have you tried a regular fast acting one instead? Also, try taking the Vitamin C at a different time than the iron.
I don't know about Vit C irritating the CD but the one bad thing about this drug is that everyone is different. Many people can't tolorate fiber but I seem to have no problems with it. Try keeping a journal of what you eat. Do you feel worse when eating high Vit C foods or is it just the pills that make you feel bad?
Also check the ingrediants to your Vit C. I learned years ago that Magnesium Stearate can upset CD and it is in a lot of vitamins. Try getting a Vit C that has almost nothing added to it and see if that helps.
Good luck and feel better.