Any idea what caused this?

Could be the collar was rubbing her, but hard to tell. Glad you are taking her to the vet to have it checked out.

Sounds like the exact same thing that my cat had. I have 3 and to make this short and sweet? It turned out to be ring worm. I was able to treat it by myself here at home by putting Blue Star ointment on it 3 times a day. This really works wonders but you want to make sure your other cat wont lick it or injest it in any way. If you put a real small amount on it and rub it in real good? That should do the trick. WASH YOR HANDS WHEN YOUR DONE. If you notice in two days time that it is getting better? Chances are that is what it was. What causes this to happen is all cats when getting in and out of their litter box tend to carry the nasty germs on their paws and under thier nails. No matter how clean you are in tending to their box. Your cat has an itch and scratches and sometimes a bit to hard where it irritates the skin. That's all it takes to start ring worm.(( If you touch it? Make sure you wash your hands)). If your other cat gets it. Treat him the same exact way. If you get it? Treat yourself the same way with the Blue star. Save yourself a vet bill and give it a try for a couple of days. If you notice it getting better like I said in a coulple of days? Continue it for a full 5 days. That should take care of your little problem and you and you cat will be happy campers once again. Good luck. Oh! You can buy it over the counter at any drug store for that matter. It's not expensive at all and goes a long long ways. Good luck to you and Boo Boo kitty. :o)

Well, it's good that she's going to the vet, so I'm not going to tell you to get her there, although this is very serious. The same thing happened to my cat on her neck. Apparently the vet said that it was a fly bite that she was allergic to. The most horrible part about it is that it ended up getting infected and a fly actually laid eggs into the hole. There were maggots in the wound and we had to take her back to the vet. They removed them and she completely healed.

saturate the sore with neosporin in the meantime...I would try this for a couple of days apply 2-3 times a day and dont be worried if she or the other cat licks the site where you put the neosporin it wont hurt them...it will work trust me my cat got into a vicious fight with a tomcat and he had a huge gaping wound and neosporin in less than a week closed the sore right up

Sounds like something fungal like a ring worm. Usually wounds like that are from fights, but since you say he/she wasn't in one.. it's probably something in the cat's system. My cat had a ringworm once and it was very itchy. He was constantly scratching it and trying to rub it on things.

Definitely go to the vet.

Cats can unfortunately easily do this kind of damage to themselves with hind claws. She may have an ear infection so that she is digging at the ear, or she may have fleas or some other irritant. She does need to see the vet.

You seem to have eliminated what I would call the most obvious causes. I suppose it could be a severe skin condition, possibly caused by an allergy. The fact that she's not purring caught my eye. Is this normal for her?

I would make sure you keep the area clean, can bathe with a saline solution - one teaspoon of salt in a cup of warm water - this will do her no harm if she licks it. As long as its not red, angry looking and she is worrying it, keep it clean til you can get her looked at by the vet

my first thought is an abcess. She got a bite, and it got infected, then a few days later it burst. if it is wetish and smells, then chances are abcess. if it is dry-ish then I'd probably think ringworm...

if the collar has any metal on it she could be having an allergic reaction to it