Personality change in a cat?

Sorry for the long post, but I have no clue if any of it could be relevant.

While I was at college, I adopted a stray 4 month old. She was the sweetest most loving cat you can imagine.. would suckle on my arm for hours on end and demand to be held all the time. She spent all her time indoors and was spayed at 6 months. When she was 18 months, I moved back home and brought her with me. She never liked the other cats, but she tolerated them. She started going outside too, although she'd just play and come straight back in. Suddenly, she disappeared for 3 days and I was shocked to find someone in the neighbourhood had taken my obviously loved and cared for cat (with a collar) to a vet to be spayed. She'd been opened up and had her ear tipped. After that I was extra careful not to let her out for long periods of time, she'd had unnecessary surgery and was just released on the streets after.. It's been a year since then and her personality has totally changed. She started disappearing for months at a time, she'd come back obviously cared for but starving.. like maybe someone had adopted her and was kicking her out when they went away on vacation or something.. The thing is, when she comes home, she's so vicious it's unbelievable. She bites, hisses and scratches at anything and everything. It's almost like she's in pain. She also "shakes" her tail a lot.. like twitching.. She did fall in to a cactus and was impaled by a spine rather badly at the base of her tail.. I removed it myself and am now thinking maybe it's left permanent damage/pain? Anyone got any ideas? I've managed to bring her in to the house and am keeping her confined in a nice quiet room for the time being but I'd really like to see her happy again

First off, how about you take her to the vet for a complete check up. Second, you know there's problems with her being outside - so don't let her out. If you must, consider building her an enclosure or teaching her to wear and walk on an harness and leash.

Did you ever find out who took off with her? The ear tipping is what is done to feral cats in a maintained colony to mark that they have been spay and neutered. To find out who had her, and has possibly been keeping her, all you would have had to do is contact vets and ask if they do spay and neuters for any feral caretakers - then ask if they recently had a female cat come in to be spay who was already spayed and match her description. Chances are they don't see too many like this (more often than not, strays are not altered).

Third, if you insist in letting her out you must get her microchipped. Since people don't always pay attention to a collar the chip might help if she ends up at the vet, yet again, or at a shelter.
Another reason you must microchip her if she's being allowed out. If this person had tested her for FIV and she had been vaccinated for that, your cat would show up positive.
I'm not sure what feral care takers and shelters response to a FIV positive cat but physically healthy cat is, but I wouldn't want to risk that someone would think she actually does have FIV.

First of all I should probably explain Shadow and I live in Northern Cyprus. I called every vet in the country - even those out of my city. Everyone denief it. I then called the rescue centre who said no way they would never take a cat with a collar. Its standard out here for all cats to be tipped when they are spayed however Shadow came with me from Turkey where my vet wouldnt tip her ear. Even if I microchip her it's not normal for vets to check for them..

She spent the night in my room last night and I'm starting to think she's schizo. She spent most of the night cuddling up to me purring and suckling on me. But suddenly she starts hissing and spitting at nothing.. As soon as she's a little more settled I shall take her to the vet for a full check up. I'm moving back to Turkey in a few weeks to our old flat even so provided she's healthy I'll take her with me. She was happy there alone with no other cats and safe with no way out other than on a harness. It's impossible in the Cyprus summer to keep a cat locked in.

Definately get her vet checked. It might be exposed to other cats she isn't happy, but better save than sorry by having a vet ensure nothing's wrong. Let us know what the vet thinks.