There are many theories associated with this phenomenon in fact. Though I believe the most credible explanation is that the body is adjusting to the changes that the Chlorus Acid(MMS mixture) is having on it. Which is likely why the effect passes over time as the body has time to adjust or compensate for the changes.
PS. some people say this is a red blood cell change, but I was never able to substantiate that in blood tests myself.
Though that doesn't mean its not the case, I'm just relaying my own experiences on that.