My view would be that your body is giving priority to producing breast milk, and so your hair gets 'fed' last (and rightfully so).
Could be a hormonal element too, as your body changes from being pregnancy to recovery/nursing, but I'm not that familiar with this aspect.
Anyway what I wanted to say, was that you may be better off looking in a pregnancy forum of Curezone, or other pregnancy related internet forums, for more answers about good nutrition for this.
Regards and good luck!