Ooooh, I know that type of cat. That's how my (now mom's) cat is. I also thought she might be a neurotic cat or something XD She can snuggle to you and you pet her, but the next second she bites for no reason. I can't really remember such wars with vet trips, but that's probably because the few trips we've made were due to her cat flu in her youth, and then few years later she decided to learn how to fly and failed, breaking a hip bone. (yes, I know, we should've secured the windows, I'm a bad cat mom

), and then there was just no reason for vet appointments. She's been running and I had to chase her all over the apartment, but she's never really attacked anyone before getting into the transporter. So maybe my cat is nicer than yours after all
I always thought I just spoiled her in some weird way, but I've read somewhere that scientists can distinguish at least 5 personalities in cats, and that let's say "neurotic loner" is one of them. I'm not entirely sure if the science was legit, but at least it made some sense XD