I think NAH.

Your feelings are understandable, as you say this is the first time you weren't invited. It's normal to feel upset when left out. Jen did give you an answer; that you don't need to attend that hangout. Your friends are allowed to hang out together, just the two of them. This is the downside to trios. If two people want to spend time alone, the other is the single person left out, which can seem mean.

Jen's responses could indicate that there's an issue, so talk to them once you've cooled down. Calmly let them know that you don't mind them hanging out on their own. You'd just appreciate honesty if there is an issue, like if you're done something to annoy them.