Mental illness? Autism?

Of course it is possible for an animal to be mentally ill, but more often fearful behaviors are caused by traumatic experiences. Is there anything that happened when she was a year old that may have caused it? It is odd that she developed normally as a kitten, and then changed suddenly without some kind of traumatic event.

Hormones can also cause skittish behavior and being in heat is extremely stressful for cats. I wonder if she would improve if spayed? She might fill out a bit, too. I've had several adult cats get much larger (not fatter!) after being spayed. Even if it does freak her out, it's a once-in-a-lifetime thing and could improve matters.

For a cat to have a disorder related to social and emotional interaction (and empathy) it would have to be a species that is more social and empathic (in that sense they can know what you're thinking) to begin with.

It sounds like you have a very small cat, due to poor genetics and there may be some other physical issues that need to be checked, but a vet should be able to do that.
She needs to be spay if she's healthy. It is not safe for such a tiny cat like that to constantly go in and out of heat and have kittens. You're worry of her stressing could lead to you losing her.
Though I'm glad you took responsibility for her kittens.

Her behavior is common in under socialized (with humans) cats. This can either happen from them being feral born or just primarily being around and bonding with one or two people. I have three in my house that are like this - they're ok with my husband and I but are deathly afraid of strangers. They were feral born, caught at ~7 weeks old.
The strangest things can stress a couple of them out to the point that they, too, will refuse to eat for a day to a day and a half. Oddly they don't go off of food from vet visits Maybe because they've had so many?
But if I make the mistake of stepping into the house with heavier shoes or boots on they're under the bed and won't eat.
This is normal, though not ideal and very frustrating sometimes. Oh, and yes, one of my girls is still kitten sized at a year and a half old - she had no problems with her spay.

BUT if you're still nervous, talk to the vet and ask how they anesthetize cats for surgery at that clinic. Let them know your concerns with her stress and small size. It may be possible to get her scheduled in as the first spay of the day and then she'd be too drugged up for the rest of the day to stress much. Also, with one of my girls I took a small box for them to put in her cage (small so it would fit). This gave her something to hide in.

I had 2 littermate brothers change from being completely social cats to very nervous cats about the age of 1. Nothing changed in the house at that time, they just became shy of people in general (other than myself) at 1 year old. These 2 boys were born to a very feral mom, however, I started handling them the moment I found them around a week old and took them away from their mom before they were 6 weeks old (not recommended to do this, but mom was very feral, it was December and getting very cold outside). I'm not sure if their early upbringing influenced their behavior, but it could have played a role.

I don't see this as a mental illness but just her personality. Mine were also afraid of trips to the vet, but I still had them neutered and had annual vet visits. Their physical health was important to monitor.

There can be significant hormone changes to a cat in the first year of their lives. I suspect that spaying her will calm her down a bit. She goes through major hormone rushes when she goes into heat - whether she tells you she's in heat or not.

Hello and welcome to TCS!

I agree with the others, that this is mostly just her personality, but you need to have her spayed. Going into heat over and over and over for four years could make any cat crazy and nervous.

Not to mention the health risks such as pyometra (deadly infection of the uterus) and uterine and breast cancers.

Please do have her spayed. As was already said, talk it over with her attending vet, there are very fast acting safe anesthesias available these days. Ask for sevoflurane.

Most cats get stressed going to the vet, but it's just a part of life that has to be dealt with.