Cat reverting to kitten behaviour?

Hi guys, I'm new here and came for some advice.

We have two cats, brother and sister, both about two years old. Our female, Jilly, is a skittish kitty: when we brought her home from the humane society, she was afraid of everything and it was a few days before she'd come out of her carrier for more than food/water and litter. Back then, she loved being petted inside the carrier, but if we pulled her out, she'd duck right back in.

After those first few days and over the past couple of years, she's stayed skittish but is otherwise friendly and happy, even with strangers, so long as she feels safe.

A couple of nights ago, she got tangled up in a plastic shopping bag we left out by accident and freaked out, running around the house in a frenzy to try to get it off of her. Now, it appears she's reverted back to her kitten behaviour, sitting behind a sofa in the basement. (She has a bad habit of licking plastic bags, so we keep them tucked in a shut closet, but this one got left out.)

Is that a common thing for a cat who's been through a trauma? For her, we imagine the bag thing was as bad as someone hurting her. She'll roll over and purr for my wife and appears to be eating and we think she's drinking (I put food and water downstairs for her; their usual eating spot is in the kitchen).

Do we just need to leave her alone to regain confidence in her safety? I think she's honestly afraid of the same thing happening again.

Thanks for any help.

I would more or less let her be, except I'd close off the bsmt entirely if possible so she's at least part of the household and easier to take care of, as well as keeping her involved (if only audibly), because otherwise she could just end up as the 'basement' cat thru habit even after she's forgotten what it was all about (I've seen it happen).

I'd thought about closing the basement off, but we keep their litter down there, so both cats are used to going to the basement regularly.

When we moved into the house about eight months ago, Jilly hid away in the basement for a couple of days then too. I left food and water down there for her then as well, and that seemed to work. I hope the same will hold true this time.

A good sign just now, though: where she's been cowering behind the sofa, I found her sitting on the back of it when I went downstairs to get something. When I'd gone down again a minute later she had hidden away again, but that's progress...

Thanks for your reply, Larke.

Jilly's a little better, and is coming out from behind the sofa on her own when we're downstairs.

Going to try bringing Jilly upstairs and closing the basement door tomorrow if she doesn't show any interest in coming up on her own.