Justin Bieber's been dropping a four letter word on Selena Gomez -- but it's not the one you're thinking. Sources close to the couple tell us J.B. and the Gomes spent the entire weekend together and it was filled with lots of "I Love You" talk on both ends. It's been less than a week since they got back together, but we're told they've already started calling each other boyfriend and girlfriend again.

As we previously reported ... Biebs and Selena went to church together last week ... they say to clean up their acts. We're told Justin is doting over Selena -- almost like a servant ... and she likes it a lot.