linenup
As the other poster mentioned, it is used for pulling out toxins as you probably know.
I have been using charcoal* for quite some time and notice from time to time pain coming after taking it. It is a strong puller of toxins and one of my key detox products, a miracle worker for me. My suspicion is that it is pulling toxins out hence the pain.
Back off the dosage and see if the pain goes away.
*I used bulk activated charcoal and take a heaping teaspoon +.