The things I have to ask are:
1. Are you eating enough leafy greens?
2. Are you taking fiber supplements?
3. Are you taking your daily vitamins WITHOUT iron?
4. And lastly, are you drinking at least 64 oz. of water (not other liquids)?
From everything I've read, some people get headaches and feel achy, almost fluish by day 5 (others, like me, don't), but it should go away within the next day or two.
If it doesn't, and you are doing all the above, you should see your doctor. You might actually have the flu!