Its quite normal...most babies born with thick or lots of hair will lose it or most of it in the first 6 months or so. You should see new stuff growing underneath, it may even be fairer, but it will grow back!!
You'll probably find that she will start getting cradle cap now too. DD never had cc until she started losing and regrowing her hair! And she had sooo much thick dark hair when she was born...now its falling and being replaced with blonde redhair! lol