Jealous of what, exactly?

"I'm jealous" is a lousy explanation if neither person knows what caused it. Was it the other person's attention? The fact that it wasn't mentioned earlier? A comparison you made in your own head? Feeling left out of a decision? Those are different problems. They won't all be solved with the same reassurance. One practical question is: "Which part of that moment stayed with you?" It's less dramatic than arguing over whether jealousy is normal. It might be more useful. You don't have to be delighted by every new feeling to take it seriously.