Your pain in your lower back could be attributed to need for liver flushing. It sounds to me like you are still in a detox mode and your kidneys may be flushing still. The detox work you did before The Master Cleanse probably created a significant amount of die-off as well and so there was an extra burden on your kidneys.
Since you have broken the fast already, I would suggest that you go buy some 100% cranberry juice. Its a little pricey and an option to that is parsley tea, or Watermelon juice. That will aid in flushing your kidneys out some more. Keep drinking fresh water by itself too. When you have resumed normal bowel function it will not be too soon to Liver Flush under the circumstances.
A good rule of thumb in the cleansing process is to detox and then clean your filters by flushing the liver/gallbladder, kidneys etc. It is a cycling process of detox-rebuild. The blood is the carrier of lymph, nutrition, oxygen and also carries the toxins away so it is important to *clean* the blood too in periods after detoxing. Eating anything green, broccoli, kale, spinach..etc...also beets and beetgreens, these things build blood and thus improve the ability of the blood to handle the burden of moving the toxins. Eating oranges and drinking fresh squeezed juice for a day fast is also a great way to oxygenate your blood. There are teas you can use, juices, like wheatgrass which are great. The point being, if you just detox, detox, detox, your body's ability to handle it all is limited and so you run the risk of having a healing crisis called the herkimer effect. Simply put...too much die-off, too fast. So, you are at a place in your cleansing routine where you must consider choices that will make a big difference in the way you feel. So after your liver flush, put some time in between your next cleanse and do some rebuilding and allow your body to integrate what you have done already.