My iphone 4 and my girlfriends seem to be sharing messages. The display shows one person typing but it is actually going back and forth between devices. Any ideas?

Question

We share and icloud and itunes account. But we have seperate carriers and phone numbers obviously.  She sends me a mesage and it comes through on my ipad.  I send her a message and it shows up on her phone as coming from her own number (no name is displayed, just the phone number). And the entire stream shows on her phone on the right hand side in blue as if she was doing all the typing. 
Then a conversation I was having with another contact showed up on her phone displayed as if she was discussing.  It was not a joint message but this contact is also one of her contacts. And the name displayed at the top was the other contact.  She could join in the conversation too but her messages showed as mine.  The other contact saw all messages but they showed as coming from my email address not imessage.  I recently did the firm ware upgrade but she did not.  Her phone says there is an error when trying to locate it.
Any messages sent to other contacts by her are only showing to the other contacts and not to me.  And they are displayed properly.
It only started today and this has never happened before. We have both power cycled a couple times and wiped out all messages to start fresh as well.  Nothing has worked so far.
Any ideas??

Answer 1

Note that if you are sharing the same AppleID when you activated iMessage the very first time, then you are literally sharing the same iMessage account.  The primary account ID for iMessage is the AppleID used to verify the service the first time - then it associates phone numbers and email addresses as aliases to that account.
One of you needs to create a new AppleID and use it JUST for iMessage, so you each have a separate and unique iMessage account to use.  So you can continue to share an AppleID for iTunes and iCloud and all your purchased content, just use different AppleIDs for iMessage.
If you need another email for AppleIDs (an AppleID is literally just an email address) just grab a gmail account to use.  In iMessage, tap on the AppleID and log off, then log back in with the new AppleID and it will activate a new iMessage account using that AppleID.