B.C.
How big is huge? A centimeter? An inch or bigger?
And where - on the top or side or back of her head?
Does she wear her hair in a tight pony tail or in tight braids all the time?
Pulling hair and keeping it under tension can cause hair loss.
It's sometimes called ballerina baldness.
It might be ringworm (a fungus) and if it is an anti fungal cream (like tinactin) will clear it up.
Does she use anything to curl or straighten her hair - curling iron, etc (did she burn her scalp?)?
How is her nutrition? Too much vitamin A or not enough Biotin can affect hair growth (and nails too).
http://children.webmd.com/guide/hair-loss-in-children