I read that vitamin C at high levels strips your body of zinc. Perhaps you should consider researching that.

What is wrong with you, is it just cold or flu or something more serious?
How are you taking the doses? Maybe you need to even up the does somewhat. It's amazing how much vitamin C the body can and does utilise. Are you taking the 20 grammes all at once, or egularly through the day? Doesing regularly through the day is best IMO. When Im sick I take 5 grammes at a time every hour, sometimes even more frequently until Im better. Drinking some orange juice with the C is also good for better absorbtion and remember drink lots of water to flush the toxins out.
Its hard for me to say because I have not been sick for longer than a few hours when using vitamin C megadoses (which I have used succesfully for 15 years now except on 2 occasions when I used calcium ascorbate - bad, bad, bad.) I hope youre felling better soon anyway!