"Jumpy" behaviour. What might be wrong with our cat?

Hi, I'm having some worries about our family's cat. My mom adopted a stray that my sister found outside about 4 years ago. They've been taking good care of her, she's always been lovely and cuddly, besides a peculiar problem that emerged soon after we adopted her. Ever since she matured, since her first heat, she's been "jumpy" for no reason. My mom let her have kittens once and then got her neutered a year or two ago. That didn't help and I think it might be getting worse over time.

To describe the behaviour... Let's say she's minding her own business, just walking around or chilling, aware of my presence, and suddenly she growls/hisses and attacks "something" behind her. Sometimes she's randomly angry at her own tail. Sometimes we walk close to her and she may suddenly growl and attack with her claws for no reason. She doesn't choose a certain person, she might attack anyone. Most of the time she's lovely and cuddly though. It has also happened in the middle of a cuddling with me or my sister.

What might be the problem? Could she be in pain? I'll try to capture a video of the behaviour but due to its random nature it's not easy. Thanks for any advice.