This is caused by either one source or two.
ONE : (and this one will always be involved) We do NOT shed our hair normally while we're pregnant. Most "shedding" while pregnant is actually breakage. Hmmm...lets see here...and average of 100 hairs a day times 40 weeks = yikes...more hair then is currently on my head. Yowza. Fortunately, it doesn't all come out at once...but it DOES come out in handfuls and clumps for weeks to months. The more that comes out at once the less time it will happen for. Humph and pfui. Silver linings and all of that. It also starts at different times for different women. Usually it starts from about a week to 3 months after birthing. It pretty much ends when it's done. Double Pfui. :P
TWO : What could be making it worse, is if you have a fairly common post pregnancy thyroid issue. By common, I mean like gestational diabetes is common. Not everyone, or even a majority of people get it. It's more then likely that you do NOT have this issue, but hey, it's a blood test.
TWOPOINTFIVE : Anorexics also lose their hair...their bodies start burning the proteins from their muscles (even heart muscle) in a desperate attempt to stay alive. Hair is pure protein, and takes a back seat to things like your immune system, and muscle repair, and blood, and milk. You're burning one whole heckovalotta calories if you're nursing. Make sure to keep your diet up with it. Lots of protiens.
Since we, as a sex, are so concerned about our hair...a trip to your doctor about it will most likely be conciliatory. <laughing> They're almost immune to us freaking out about it. If it IS thyroid related though, a blood test will show it. If it's not thyroid or diet related the best advice is the same on how to keep your hair health normally. Wash, condition, brush, & eat lots of protein in a healthy diet.
Good Luck, and this too shall pass.