Why did my Facebook message go from "seen" to "sent"?

[pri✍]

Sent is just as it means the message was sent at xx.xx hrs. "Seen at xx.xx" means the recipient opened that pm at that time.

However, we have the option to Mark Unread the last message, thus delete the "Seen at". It can be found as the first option under Action on top, or on the right of the name in the blue box on the left column. (talking from the view of FB full site)