Cat Bites

I finally moved my cat from my parents home to my apartment over a year ago, she was having to live in the basement because they had a dog that would chase her all day and it was very miserable for her. I love my cat and she is very comfortable with me - she sits on my chest and hangs out with me all day. She’s always had a bit of a biting problem, and I think it’s just part of her life experience at this point (she’s 9) but recently she has been biting really hard when she doesn’t get what she wants right away. She consistently draws blood, and will switch very quickly from purring and sitting on my chest to biting me really hard. I’ve tried to mitigate the harm I am inflicted by sending her to “timeout” as a constant bad thing. That’s where I just put her in a room I’m not in. If I’m in my room she goes in my living room and vice versa. I always let her out when she scratches so she’s never in timeout long, but even though I know she now understands what “timeout” means she still bites like crazy. (I’ve tried other things in the past but she’s always learned the wrong skill from it all - for example saying “no” after she bites just made her learn to sit down after she bites but not stop biting)

Right now im wondering if she is under-stimulated, I live in a small apartment, and now that the novice of it all has left her she’s getting antsy? Or maybe she is sick and it’s getting her aggravated ? Outside of the behavior intensity nothing about her has changed. I can take her to the vet but I live in an expensive city, and if it truly is just behavioral and not a medical thing, I can’t really afford the unneeded bill.

Does anyone know what I can do? I’m planning a slow introduction to a neighbor’s cat for “cat-play dates” in the future to hopefully help if she’s under stimulated but that’s about it..