I'm poly, but I've had that weird feeling too. The "Shouldn't I be jealous?" kind of thing.
I'm used to hearing about jealousy from our monogamous society. I also read about polyamory and it often said "we're still jealous, we just work at it".
So I expected to be jealous. But that hasn't happened. I'm only happy when Rag talks about J, when stuff has happened, I try to help them get closer, the idea of them having sex makes me very happy...
The main thing is considering I was Rag's only experience before, I keep thinking "shouldn't I feel jealous about not being the only one anymore?" but that just doesn't happen.
I think it's because I'm there. I get to see it, hear about it, take part in it. It would probably be more painful to hear about it after the fact or from someone else and know Rag was hiding it from me, or something.