This so happened to me too! Unfortunately it's still continuing and my daughter is 21 months now. It's horribly stressful. We women take pride in our hair!
I was first told by my doctor that it was the usual hormonal change that happens to all women. Your body is trying to "get back to normal" and regulate itself again. I get that. It happens. But this is not a continuing "normal" thing by the time your child is a year or older (especially if you have stopped breast feeding).
After you have ridden this out, meaning if it persists and/or worsens after your child turns a year you may want to seek another opinion. It could be a thyroid issue, stress-related, etc. Blood work should clear that up. If it doesn't...then you and I are in the same boat. If your blood work is clear the only explanation a doctor will give you (at least in my experience so far) is that it is female pattern baldness.
If female pattern baldness is the case, you are only able to treat it with a specific Rogaine geared towards women. But be aware that you cannot treat your hair with this product if you are pregnant, breastfeeding or trying to conceive.
Good luck! I feel your pain! : )