Mercury can come from a lot of different sources, so it's hard to know what was the cause of your overload. If you body is poor at chelating than eating a lot of a mercury-heavy fish like tuna could be enough to make you sick.
The side effects with cilantro would likely be worse than with DMSA. Keep in mind that negative reactions are not always a bad thing. In fact you can look at them as a sign that the mercury is being removed. The reactions will become less and less as the metal is reduced in your body.
Drinking a lot of water and going to a sauna can be helpful in getting rid of the metal and reducing side effects.