iPhone 4 Mail Allegedly Hijacked by SMS

I received a very strange spam text message yesterday (NOT the black box) that had a spam message, a link and came from a very strange phone number. I immediately deleted this text message and did NOT go to the link.
About 10 minutes later, my gmail account starting spamming people in my contacts list seemingly from my iPhone.
Now, before I go any further:
I do not log into my e-mail account anywhere except from my iPhone and my Mac at home.I use a characteristically strong password, so I know my password was not guessed or hacked.
I say that my iPhone was hijacked because I was chatting with a friend at the time who informed me that they just received a spam message from me so I immediately changed my gmail password. After I changed the password, I deleted the mail account from my iPhone yet messages continued to be sent (even AFTER I had changed my password).
Eventually I turned my phone OFF and that stopped the messages. I rebooted the phone and the messages have not been sent since the reboot. Can anyone tell me what happened, please? And if Apple has some sort of fix for this in the works?
Thanks!
Kiffy
iPhone 4 iOS 4