Is paranoia normal for grieving cats

I had 3 kitties to love, then Grizz (11yr old MaineCoon) passed away. Now one of the two remaining cats has become paranoid in her own loving home. She slinks around and acts like something is going to pop out on her. I understand that cats grieve, but this off-the-chart paranoia is
making me very sad for her. I don’t want her to go through that everyday. I’m wondering if maybe she can see ghost Grizz and she thinks he might pop up out of nowhere or is this a “normal” grieving cat behavior. Nothing else has changed in our environment. Grizz passed away on April 3, 2024, were now in September 2024 and I don’t see any improvement. I’ve even used kitty-safe essential oils, but that didn’t help either. My second other cat is doing fine…just not her. Does anyone have any advice or has had the same experience? Please Help!