Try external vit D3, magnesium most important
If you are using distilled water, switch over to good clear groundwater or springwater from your store. Distilled water steals minerals from the body.
Try adding a little salt to your water to balance the level of salt expended in flushing out the toxins from your soft tissues. If you are focusing on water only, then soak for a good hour in
Seasalt mixed with epsom salt.
Magnesium is vital in keeping your intestines lubricated and relaxed, and a major tensing is a muscle cramp down there.
If you have not had your vitamin D3 level tested recently, get it checked. If you are in the states and it is below 80, you need to start vitamin D right away, and read about it's cofactors, magnesium, boron (stops the kidneys from spilling calcium and magnesium), zinc, and vitamin A and k2 from fermented foods.
You can absorb vitamin D dermally if you are sticking right to your fast, but intense pain in the gut is the neurons in the intestinal lining also crying out for what they need. Vitamin D3 together with magnesium gives an immediate pain relief and is ratherten times more powerful than asprin, tylenol, etc, I've seen it work three times at treatment doses. (50,000 iu within 1-2 hours with water).
For an external application, buy oil-suspended vitaminD3 (healthy origins and doctor's best do this), and pop the capsule with a needle, and squeeze the oil onto a good, soft bit of skin like the armpit or groinal area, where the skin will absorb it very well. You can choose yourself how much you use.
Do not buy vitamin D2, it is a byproduct of fungal spores and is rather ineffective.
Blessings, -Mew