<human>: My SO [Male, 19] recently started talking to an old friend of his younger sister. Me [Female, 18] and him were perfectly fine, our relationship at 6 months now. I was fine with them catching up, then they began to talk more. Over the past five days, he has barely been talking to me, and told me that he feels like me and him "talk to much and have nothing to talk about anymore". I gave him his space, hoping he'd come back to me as always.

Last night, he called me and acted like a was a total stranger. He made some small talk and then brought up the girl. He told me that she likes him a lot and seems really sweet and innocent. He's really interested in talking to her and said he's begining to like her too. Then he goes on about how his love for me wasn't the same as being in love with me, and how she's the one and he feels like he'll end up marrying her.

Normally I'd be shocked and end it, but here's the catch. He's played this practical joke on me before when I was first introduced to his best friend (female). I also find it hard to believe that he'd talk to this girl and within a week he'd move on so quickly. Just last week we were falling asleep on the phone together and he poured his heart out about how he loved me and wouldn't want to see himself with another girl. He's also kept me up all night when he was drunk and confirmed it, in a way.

Should I be worried? How can I get her to back off? Any advice would be wonderful.

Can you provide a brief overview of the text?
<bot>: Boyfriend is talking to an old friend over the past week and told me she might be the one. He's played a practical joke like this before but I'm weary. Advice?