Can't remember where I read this but it is not unusual for moms to lose
significant quantities of hair starting 3 months postpartum.  I wish I could
remember the details but it is to do with the hair follicles not going into
their rest phase during pregnancy -- hence the lovely thick hair so many
pregnant women enjoy.  With the dramatic drop in estrogen at birth the hair
follicles that are overdue to rest all do so.  It takes three months then the
hair that was growing in these now resting follicles falls out -- in great
quantities for some women.  By the end of a year all is stable and hair is
back in normal rest/growth cycles.
Joanna Koch IBCLC in Los Altos, CA