It's postpartum Telogen Effluvium. That happened to me three months after the birth of my first. When I was pregnant, I lost much, much less hair than I normally did, due to high estrogen and progesterone levels during pregnancy, which causes hair to remain in the anagen phase longer. God, my hair never looked so good! Then it started falling out like you could not imagine. My vacuum literally got clogged (I had carpets at the time) and shut off due to so many hairs all over the place. Talk about panic. Anyway, I think it lasted maybe two months (that was over 20 years ago so it's hard to remember), and all of it grew back.