Asking is totally fine - it's taken me a bit to figure out how to answer you with something that explains my feelings right now.
The scenario that drives my bad attitude is that I'm emotionally vulnerable. An old story, but a small one I can share, for example. Sylvia was working and unavailable, and I had an unexpected gap in my schedule, so Joshua and I could meet up for a brief lunch. We got to talk, and enjoy it, but it triggered her possessiveness and became something that she'd vent about, including to me, for quite a while. It was a problem that I got something "extra" with her primary - even if it didn't take away from her time and attention - which meant a lot to me given how limited our time was. To feel like I needed to defend it, and have something sweet and special to me become a source of drama makes me feel emotionally vulnerable to a person I didn't choose.
It's a bigger emotional drama if it involves a gift, or a more involved date activity, or something sexual. I'm not confrontational if I can help it and don't do well if I'm caught by surprise, so I've been carrying a feeling of needing to be on the defensive constantly since our last bit of drama about two months ago. That defensiveness is affecting my relationships now - making it hard for me to be generous about the good times my lovers have with her because I'm suspicious of her motives, or want to retaliate for feeling hurt, or feeling that my openness with my partners isn't reciprocated respectfully.
I really don't want to paint my OSO as a monster because she isn't. She's smart and loving and fun and brave and brings a lot of joy to people I care about. She knows she struggles with her reactions and genuinely works on improving her handling of them and owns up to her actions. We're just very, very different in temperament and personal history and approach things in incompatible ways, and recently I can't seem to get back to shaking it off and moving on the way I've been able to before.