Just recently the hairs on my legs has started to grow very long espicially up the back of my legs above the knee where I've never had hair growing before

Maybe it's hormonal imbalance?

There are a lot of factors that can cause excessive hair growth in women. Some women have more hair than others due to heredity, stress, hormonal changes or medications. If you're concerned about the hair on your legs, you can use depilatory creams, shave your legs, or have them waxed.