Transformer Girls

Gracie Carson is a normal twelve year old girl, except for the fact the she is very smart and good at almost everything. The only thing she dreads is taking care of her little sister who gets everything she wants. Then one day, on her thirteen birthday she has the sudden urge to go and swim in the ocean close to her house. After getting in the water she looks down and finds that she has sprouted a turquoise blue tail. Horrified she swims back to land and the tail immediately disappears. Soon after, she transforms into other mystical creatures, which in transforming into a fairy she finds that her best friend, Bethany Jackson, is also having the same thing happen to her. Can they control their new abilities and hide their secret from their parents and other people? Or will they find themselves having to be dead to the world forever?

Warning:
Please don't copy my idea, if I accidentally have a similar story to one of your stories please tell me in comments and tell me what story it is similar to.