Get the Carol Vorderman Detox book. Basically you have to eliminate certain foods for a few weeks- foods that don't do us any favours and that some people are intolerant to.
Wheat, dairy, meat, caffiene, alcohol and sugar. You can make perfectly nice meals without these things. Once you start this, you may get headaches, a bad "cold" and feel exhausted, but this is jjust your body having a clear out and all the toxins coming out! Stick with it and after a few weeks you will feel SO much better. You can then, if you like, reintroduce one food type at a time, and see how you feel. It could be that eg- wheat makes you bloated, dairy makes you congested etc. I do these from time to time and swear by them.