I think it's normal to lose hair after baby is born. I'm sure someone could explain it better, but from what I understand, pregnancy hormones cause your hair to grow thicker. After you're not pregnant anymore, the decrease in hormones causes you to lose some hair. I think by the time dd was four months I was finally not losing tons of hair every day in the shower.
If your hair loss was really excessive, it maybe could be due to losing too much weight. I'm not sure about that, though.