It could be one or a combination of three things, at least that I'm aware of.
1. You don't tolerate DMSA that well. A number of people, such as boronia and TK who post here, just do not react well with this drug.
2. You're a fast metabolizer, so you need to take it every 3 hours.
3. You started off with a little higher dose than your body can handle.
You may want to try taking a smaller amount of DMSA (say 10 mg) every 3 hours. If you have problems with that, then you probably have an intolerance to DMSA. ALA is really the most important thing to take for mercury poisoning, so you can stick with ALA alone or look into ALA + DMPS. Unfortunately, DMPS costs maybe 10x as much as DMSA, and you need a doctor's prescription for it.