Hi,
The phenomenon you noticed is quite normal.  Like most of the changes that occur during pregnancy, it is also temporary and affects about 40 to 50% of the women.                              
This excessive hair falling lasts from one to five months following pregnancy.
During pregnancy there is an increase in the level of estrogen hormones. This estrogen hormone causes hair to remain in the growing phase and stimulate the growth of your hair. Also, the rise in hormones during pregnancy keeps hair that would have normally fallen out.
After delivery, the hormones return to normal levels, and allow the hair to fall out and return to its normal cycle.
The normal hair loss that had been delayed because of pregnancy tends to fall out at the same time and is noticed as excessive loss of hair.
The hair loss you may experience could worsen over the next four months and persist for several more as your hair follicles rejuvenate themselves.
You are correct in taking the prenatal capsules which contain vitamins, calcium while nursing.  This helps in maintaining your health.
As noted before, this hair loss is temporary. Hair loss is usually replaced with fresh hair within six to twelve months.
Other causes for excessive hair loss should be ruled out. Your primary health care giver should be able to help you here.

References:

1.  Diseases on Hair Follicles Leading to Hair Loss Part I: Nonscarring Alopecias
2.  Primary Care Dermatology Roundtable: Common Dermatologic Conditions Seen in Primary Care Practice (Clinical Review)