Why Does Our Cat Shiver?

Not to try and make you sound ignorant, but your cat is probably cold. If the cat is older (like 11+ years) or very young (0-2 years) it could just not be adapting to the changing seasons as well. I have a screened-in florida room off the back of my house that my cats LOVE (I refuse to actually let them outside, but with this they get some fresh air) and when I open the sliding door for them in the winter, they always give a good shiver or two before bolting outside after imaginary birds. If the shivering doesn't stop as the weather gets warmer, it might be something more serious, but I wouldn't worry too much about it now.

It's probably the cat grass that you're growing in the house. If it's new to what the cat eats, and she isn't use to eating "artificial" grass, then it stands to reason that this is what's causing it.
Use the grass that's been growing outside,,it has all the natural elements that contribute to a healthier veggie for the cat. If it doesn't stop after that, then get her to the vet. It could be something neurological. (I hate to say tumour, but it could be something life threatening.) Hope this helps.