OK, Blume, I misunderstood. My error. I'm really glad you've talked about things together, that is SO important!
Here's another thought: To some people sharing sexual intimacy is not threatening. It doesn't trigger jealousy. But emotional intimacy -- hearing or imagining your partner saying "I love you" to someone else -- is devastating.
To read her writing "you're the only one" to someone else certainly sounds like a hard thing to see! I can understand how it would hit you.
Talk to her about that exact thing.
Of course not in a threatening or complaining way, but in the "darling, I felt some really strong emotions when I read that, can you help me understand them" kind of way.
From your posts I'm not sure if she knows you read the MSN comments. If she doesn't know then that makes it harder, because she might feel like you were snooping.
Without knowing a lot more about both of you I can't even guess how you might approach the subject, I don't know if she would be likely to be touchy about it or if she's very easygoing. Those things aren't easily understood from internet forum posts!
Which leads me to my usual observation: everything I've written is guesswork based on a little information. You are the guy who really knows what it's like in your shoes, so if I've written something that seems wrong then disregard it!
It's not unusual to be bushwhacked by feelings you didn't expect. You are FAR from alone!