hey T.,
Thrush is more than likely a cause of too much Candida in the body. This is somewhat of a contraversial topic as most doctors don't believe in systemic Candida. It can be passed back and forth from mother to nursing infant.....you need to completely STOP eating ALL sugars and simple, refined carbs, like white breads. And if you supplement with formula, try to not use it; exclusively breastfeed with a good, clean organic, mostly protein diet, without sugars.
I have had systemic Candida problems for several years now and it is very difficult to rid of the yeast; it also comes back easily!
Research and locate a quality probiotic for you to take, which will pass onto your son as you breastfeed. It is completely safe for both of you to take a probiotic. Also, try to eat plain, unsweetened, organic yogurt. You can also spread a little layer on your nipples before and after breastfeeding.
Pam