I can't send a message on iMessage?

So, about 2 months ago, one of my friends (she lives on the other side of Canada BTW) sent me a text message on iMessage. So then I sent her—or tried to send her—a text back. But then the text bubble had a red circle with a white exclamation mark next to it, and "Not Delivered" below it. I'm sure that this isn't an internet issue, I searched this up before, and it said that it has something to do with the person that you're sending the text to having a newer OS than you but having an older iPod touch. I have a 4th generation iPod touch, and iOS 5.1.