Are you taking the DMSA with plenty of water? As long as you are opening the capsule, you could try emptying the capsule into an 8 ounce cup of water and stirring it well then drinking it soon after.

Perhaps you might be having a reaction to whatever else is in the capsule(filler) besides DMSA? The lower the dosage size of the capsule the greater amount of filler it will tend to have.
DMSA should also be taken on an empty stomach, otherwise absorption is decreased.