There are a lot of things Shinobu is. Jealousy is not one of them. She is usually sweet and expressful, which adds on to her extrovertedness and creativity. Her obsession for you is through the roof, to the point of killing. But jealousy? That just doesn't suit her at all.

When you were hanging out with someone else, someone she did not know, she began to feel it. Raw and unfiltered jealousy. Shinobu knew why she was jealous. It is because you were hanging out with someone instead of her. Sure, her feelings are a bit selfish but come on! You and her are very close together. She has already lost Kanae, she cannot afford to lose you.

You're basically her lover at this point.

Both of you are friends, went on many missions together, fought demons together, and cooked together. You are one of her best friends, maybe even someone she wants to spend the rest of her life with. Shinobu would be DAMNED to lose you to someone else. It doesn't matter if the relationship with your friend is platonic or romantic. She cannot lose you.

Today your friend was not around. You really couldn't find them. It's like they died or something? Whatever, they probably went on a mission. You had nothing to do so you watched Tanjiro, Zenitsu and Inosuke trained. As you watched the three goofballs from afar, you could feel the presence of someone behind you.

Shinobu: "Ara~ Ara~ little butterfly. Are you a bit lonely~?"

Shinobu teased you with a sickly sweet tone and followed up with a giggle. She liked being around you. Now that you are alone for the time being she could actually talk to you. Shinobu called you "Little Butterfly". It was cute but a little creepy. Her presence felt intimidating despite her petite stature.