Besides the need for very healthy food your body wants at this time, I would be very concerned about the toxins you will be releasing (through your skin, breath, bowels and milk).
When I was nursing (14.5 months) my bones lost alot of calcium.
My back would sieze up with the slightest movement and I felt very weak.
Of course, I hadn't gotten educated about healthy eating enough to curb this problem.
Breastfeeding will deplete your body of calcium (your body knows the baby needs it more than you do) but once you stop, your bones will regenerate and become even stronger than if you had never breastfed at all.
If I were you, I would eat the healthiest food possible (organic) NOW.
Start training your taste buds now for what is to come after you finish breastfeeding and then the MC.
Eat many high calcium foods (yet don't always eat the high fat ones).
Almonds (3oz has 214mg)
Carob -- I like it better than chocolate now (3oz has 315mg)
Cheddar (3oz has 682mg) & Swiss cheese (3oz has 841mg)
Kelp (3oz has 990mg)
Seaweed (3oz has 1710mg)
Bok choy (a cabbage)
Salmon and sardines (canned, with bones)
Try garnishing meals with Parmesan cheese and/or sesame seeds.
And no soft drinks at all (they deplete your bones).
Don't really worry about gaining weight during this time.
I lost 10 pounds I had before I got pregnant.
Just hold off for a while before doing the MC.
Your baby will repay you with fewer trips to the doctor and a healthier life (for both of you).
And later when you finish the Master-Cleanse (and I strongly recommend it along with alot of other flushes at this website) your body will be clean and strong and your next pregnancy will go much easier.