You don't completely stop shedding hair when pregnant, but the volume of hairs shed is dramatically reduced. Once the pregnancy hormones leave your system, you begin to shed the extra hair again which makes you feel like you're losing a ton.
I've lost almost all my hair during chemo, and it sucks! I'm 18 months out of treatment, and my hair just started shedding again a few months ago. I'd forgotten how much I hate cleaning dark, Italian hair off the floor of the bathroom and the sink.
The amount and time of your hair loss likely dependent on your body's ability to eliminate the excess pregnancy hormones. For me, it only lasted a few weeks with my son, but I was only 11 weeks postpartum with my daughter when I started chemo, so I don't really have a good point of reference.