My phone acts like it's my mom' phone

My mother used to have an iPhone 3GS but then gave it to me. It had MY number and all My contacts on it. The phone had my mom's apple ID on it though and I had to use her password to download apps and stuff. Today we went to the AT&T store and I upgraded to an iPhone 4. My iPhone 4 has all my contacts on it but it acts like its my mom's phone. When I texted my brother it showed that he got the text from my mom. I then texted my mom and the text message was sent to myself as well as mom's iPhone. I am so confused. My iPhone4 has my number but when I text people, the text comes from my mom's phone. It's as if my mom's phone and my phone are the same, but how can that be? Can there be two phones with the same number? I don't understand how when I text my mom it sends the message to her but then I get the same message that looks like my mom sent it to me.

Sorry if this is confusing but I am very confused myself! Please help if you can.