Here's the deal: They tested a 120-130 milliliter sample of breast milk from this woman and discovered an amount of I-131 that is equivalent to a 36.3 Bq per kilogram concentration.
The safety limit is 100 becquerels per kg for tap water consumption by infants under 1 year old, but that is besides the point - if we evaluate the official I-131 readings in water from atmc.jp, officials will be hard pressed to explain how she accumulated even this amount in her breast milk. From their website:
Based on their data, I-131 levels have been virtually undetectable since 4/8 and were only reading 0.30 Bq per kg as of 4/19. Assuming evenly distributed isotope concentrations throughout her bodily fluids, this woman would have to drink 121 liters of water in a day and not excrete any of the I-131 for levels to be this high.
Since this is obviously not the case, there are four possibilities that would explain the excessively high reading - 1) the source of the I-131 is mostly in air, not water 2) she recently arrived in Chiba from another area with much higher I-131 concentrations in water (not likely) 3) she got nailed with a much higher dose early on in the crisis, or 4) these atmc.jp numbers are not even close to being accurate.
Remember, I-131 has a short half-life (8 days) and bleeds activity at a rate of about 8% per day. Even at the height of the crisis, tap water tested in Tokyo rose above safety limits for just a few days time and never exceeded 250Bq/kg.
Edit: The I-131 biological half-life is as follows - thyroid - 100 days, bone - 14 days, and kidney, spleen, and reproductive organs - 7 days. I can't find info on mammary glands. It's possible that this woman took in a lot of isotope at another location. Health officials need a detailed history of her whereabouts over the last few weeks to determine if she was exposed to a new batch recently or if she was highly dosed from a few weeks back.
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