why does my iPhone 5s lose charge when its off

Why does my iPhone 5s lose charge when its turned off???????? Earlier today i took my iPhone 5s off charge at 100% and i went back to it in an hour or two and it was at 75%!!!!! whats wrong with it. And it takes ages to charge my iPhone 5s!!!! And i i got my iPhone for christmas in 2015!!!!! PLEASE HELP ME I LOVE MY IPHONE