This early in a fast you may very well be congested in your colon. Have you done a successful swf? If not... do one! Flush your colon well, fast through and your problem should go away. A plain water enema can be a good option to get things cleared out.
Gas is a sign of poor digestion. Something you are eating is not being efficiently digested. Wrong combinations? Sometimes raw foods do not sit well with people. There can be a lot of things, and this cleanse can help you find out what.
Do your ten day MC, break by the book, then a little at a time start adding your normal foods back into your diet, taking note of what is causing you trouble.