Congrats on another sign of success from the MC. Your mentruation is another way your body has to eliminate toxins. My flow was also heavier while on the MC, and I was told this same thing. After researching on the internet, I realized it makes sense. Isn't it "wonderful" that we women have an additional way to see proof that our cleanses are working....
I've heard that after awhile on raw food only, you can see monthly proof that your body is getting very clean by the amount/color/smell of your monthly elimination. This is supposed to be a good clue also about when you're having another "cleansing event".
Personally, I found that if I try to improve my health on the day before or the first few days of my period, I CANNOT control my eating. But if I make the change a few days or more before, or a week after I start my period, I have GREAT success. Hopefully this will hold true for you!