As always, start by trying to communicate with your kirlia. Kirlia are very sensitive psychic-types, and they can pick up on things that might be wrong—or transmit how they feel, for that matter. Sit down with your kirlia and ask her what’s on her mind, so to speak.
Only then can you proceed forward to resolve the issue, as it could be any number of things, from a fear of your new steenee to jealousy to even warning you about a potential latent allergy to grass-types you might not be aware of. Each possibility has its own solution, so in short, it’s best to narrow things down before trying anything with your kirlia and steenee.
As a note of reassurance, most possibilities may be resolved with a simple conversation with your kirlia and possibly by gradually introducing her to your steenee. If this is indeed the case, do so slowly and gradually, in a setting both your kirlia and steenee can be comfortable with. As both are human-like pokémon, they’ll be more active when it comes to vocal communication with each other, so be prepared to mediate any arguments or fights.
Best of luck, anonymous!