That does not sound like a DMSA specific symptom. Fibro is a not so rare mercury symptom and so can be intensified while on round, especially if not treated adequately. Cutler mentions Fibro in his AI book
Remember Mg is on Cutler's short list of supplement to take! It can be involved in Fibro.
Many people need more fluids and electrolytes while on round, especially in warm months.
Of course that person would be wise to continue to work with her doctor. Obviously not all symptoms are due to heavy metals, however, I would not be surprised if this one is due to mobilization of Hg by a chelator in an inadequately supported person.