A White Lie For No Reason

I'm in my early 50s and we've been openly non-monogamous for years. I thought I was past the whole jealousy thing, but apparently not. My partner has a pretty close friend from work who they talk to quite often on the phone, even sending texts back and forth sometimes. I found out about it accidentally one day when she left her phone with me while we were hanging out at my place. Curiosity got the better of me, and I ended up reading some of their messages. There was nothing untoward going on between them, they're just good friends, but still, something in me felt a little bit threatened. To reassure myself that there wasn't anything going on with this other guy, I even called him under the guise of being my partner to set up a date night for her and him. They agreed happily. The lie was so small, yet it made such a mess of things in my head. Was jealousy really something I still had to contend with, decades into my relationships? It's not as if anything happened between them; they just have a rapport that goes beyond platonic interactions sometimes. But knowing I lied about nothing at all is gnawing on me now.