Having hair on the chin, breast and other male-pattern hair growth is collectively known as hirsutism. Unfortunately, it's actually quite common, affecting between 5% and 10% of women.
Hirsutism is caused be a hormonal imbalance in the body. All women have what are traditionally known as 'female hormones' as well as what are known as 'male hormones'. Hirsutism is a result of having increased amounts of male hormones (androgens) circulating in the body, and having hair follicles that are more sensitive to the hormone. Having too much androgen is one of the most common endocrine disorders in women.
Reference: Androgen Excess, Medscape; Abdel-Rahman, MY et al. August 5 2010