This is Day 3 on your fast. Usually the digestive system takes about 3 or 4 days to shut down completely. When you stop eating this gives your body a message to begin triggering the detox. This could be what's happening with you now. You could be having what's called a healing crisis. This is where the body feels ill, nausea, cramping, blocked up sinuses, rashes, headaches, cold and flu symptoms, acne, boils... etc... this is because poisons are entering your blood stream and leaving your body. These things come and go relatively quickly. Your best bet is to ride it out. Drink lots of water, drink lots of juices, make sure you have lots of GREENS in those juices. REST REST REST when you're feeling badly.
Another thing to consider is on a fast, you're not getting fiber, which will clog you up. You need to make sure you're drinking herbal laxative teas so your colon will continue to move. Many juice fasters do enemas, have colonics, take colon/bowel cleansers so to make sure the bowels move consistently every day, a few times a day. AusJulie can give you more details here... this is not my forte. But anyway, if your bowels are not moving then the toxins are building up in your system. Gas is a sign of stopped up bowels. If your bowels are not moving then perhaps consider using a bowel cleanser. CureZone recommends several. You might want to peruse the Bowel Cleanser forums. I just ordered Oxy Powder www.oxypowder.com as it is supposed to be superb at cleaning the bowels without the cramps. The bottom line is it is crucial your bowels move consistently while fasting.
Why not consider using a liquid supplement instead of solid vitamins? I am assuming when you say supplement you are referring to vitamins. Usually people fasting don't need to take the supplements, this is why it's also important to make sure your juices include lots of greens and root vegetables. Fruits are good, but should be somewhat limited as opposed to the veggie juices. You might want to read some of my blogs as they discuss liquid supplements as well as other tips and ideas while fasting.
This is really the best answer I can give you with the limited information I have of you and your predicament. I sincerely hope you feel better soon, if not tomorrow morning when you wake up after a good night's rest.
I wish you the best,