cat suddenly became violent

I'm very concerned about the behavior of one of my stray cats named Lucy. I found Lucy about a year ago and since then,  I've been taking care of her. I feed her daily and on extreme weather days, I take her into my garage. Other than that, she's pretty much an outdoor cat. I have taken her to the vet and he informed me that she had already been spayed by her previous owner. Lucy is a pretty chilled cat. She's very low energy and just likes to sleep all day. There are times where she'd get into fights with other strays in the neighborhood but nothing too violet. She doesn't get along with my two other indoor cats but again, she pretty much keeps to herself. 

One day, I took one of cats I've been feeding to get spayed. The vet told me I had to take care of this cat for two weeks until her wounds heal. So I took this cat into my garage. Lucy noticed this and at first, she acted very curious. Then today, she snuck into the garage when I was cleaning it. Now she doesn't want to leave. I tried offering food and toys but she's being very stubborn. She has also started to act violently. She would hiss and growl at me and my sister and my other indoor cats ( their litter box is in the garage) as well as the other stray cat that just got spayed. Lucy got into a fight with one of my indoor cats just a few hours ago. I don't know what's gotten into her. I'm afraid to touch Lucy because she'd actually show her teeth and yeah, I don't think it's safe for me. I'm pretty sure I can get her out tomorrow ( by withholding food for the night) but my question is, should I be concerned about Lucy's behavior? Like does she have some neurological disorder? I'm so surprised by her behavior.  I think I've tried 4 times now to take her out of the garage but she's just so scary to deal with. She's also sleeping/sitting on my sister's printer ( she uses it for her job and it's worth thousands of dollars) so I'm worried that if it gets broken, I'd have to pay for it too. I'm so upset at Lucy right now I can't believe she'd so this to me. Any suggestions to this problem would help. T hank you in advance. 

It sounds like she is being territorial, although she's being awfully aggressive about it.

If she hasn't had her shots, you should get her to the vet as soon as possible to determine if she's caught something.

I don't want to frighten you, but a vet needs to check her, so I would trap her as soon as possible.

That will at least allay any fears and help you determine if she's just being overly territorial.

Thank you for all you are doing for all of these cats.

Ondine has very good advice. I agree, it sounds territorial to me.

It would be best to get her to the vet if at all possible to get her checked out to make sure she hasn't caught something. And to get her shots if she isn't up to date but ask the vet as to what he or she would recommend and go with what the vet thinks is best.

You can try to put something on the expensive printer like tin foil (as long as tin foil does not damage the computer, I don't know if it would or wouldn't). Or maybe hang something above the printer or put a broom near it or something that would cause the cat to not want to go up on it. I am not sure but try to think of something to get her away from it without doing harm to the printer. Was the printer used (warm) just before the cat went on it?

Just be careful not to provoke her into attacking you or someone else. Watch the warning signs, hissing, growling, ears pinned back, tail moving rapidly, etc and if you see it just back off. Be confident and try not to panic when backing off as cats can sense negative energy.

Hope this helps a little.