did someone hack my iphone?

ok so im wondering can someone hack your iphone? me and my mother both have iphones on one apple account along with a few ipods. So recently someone has hacked into my apple account and spent over 300 dollars on my apple account which is connected to my moms bank account but i also noticed that my moms imessage hasnt been working andher phone has been moving really slow and freezing alot and her email hasnt been working either and the purchases are all on games she plays on her phone. my mom hardly ever uses her phone though and the purchases arent being purchased on her game account so we know its not her doing it on accident. we wouldnt even have noticed if the money didnt depleat so fast on her bank card yesturday. so i was wondering could someone have hacked into my moms phone? and how did they do that, also how can i get it off. oh yeah and my moms never jail broken her phone or anything and we both have the iphone 4.