Hmm... thats interesting about the dumping you are reporting. I've been experiencing similar effects, though I've not taken DMSA yet. I'm not so sure if it's heavy metals coming out or not, but for the past 2 weeks or so up till 2 day ago I've been experiencing these massive headaches (migraines?) on a daily basis, several times a day. It was horrible and incapacitating headaches that I've not experienced for nearly a year. Perhaps there is something to this "seasonal cleansing" you are talking about.
Right now, I'm just glad the headaches finally stopped.