Jamie, don't give up the Vit C.
You should have taken SLOW RELEASE tablets rather than the fizzy laxative type. That bloke Linus Pauling (mentioned by Dr B in this thread) may have been a bit extreme but you can't dismiss entirely someone who won 2 Nobel prizes.
Most alternative health practitioners extol the virtues of vitamin C and unless you already live on a diet of fruit you should get benefits in terms of less colds and viruses. I wont bore you with details - there is tons of stuff on the www.
Personally I take about 3 to 4g of Vit C per day if I feel that there is risk of a cold. I never take less than 2g per day. Your body can't store Vit C so a sudden massive dose is not much use.
I suggest you try taking up to 4 x 1g slow release tablets at intervals during the day with food. If you get the shits you've taken too much (your last the problem was more likely that it was the wrong form than the amount as 1g is only excessive as a sudden hit). Get well.