It is very common, especially with candida die off.

It is important to drink extra water during chelation, and to drink a full cup of water when you take ALA or DMSA. Since much of the mercury is excreted in the bile, it is also important to avoid constipation while chelating. Eating extra soluble fiber helps, as it not only prevents constipation(which is very common for people with mercury toxicity), but also allows less of the bile to be recycled, and therefore less mercury to be reabsorbed.