Cyst on cats head

Question
I adopted a stray cat from our neighborhood in January 2012, almost a year ago. She gave birth in February and in May we neutered her. Through winter she was an indoor cat, but when summer came she wanted so desperatly out so during summer she was mostly out, coming in only for food. When i got back from vacation, i saw she has something like a cyst on her right cheek. I took her to the vet immediatly, and he said he can't know for sure what is it, but he thinks it might be something stuck in, like a piece of wood, or maybe some insect bite her. She got some injections. Two weeks after, it started again. Then, the vet said she needs surgery to open that and clean it. He said after that it was full of pus, but he didn't find anything else. That was in August. Month later, it started growing again. We took her to get an X-ray. We saw a bullet in her head. She had an emergency surgery to get a bullet out of her head. She recovered, but that cyst came back in November. Since November, my cat would scratch it and the pus would come out, the vet will desinficate the open wound and she would be okay for about 2 weeks and then it will repeat. I am devastated. The cyst is about 3 cm wide and about the same long. It is soft, and if you touch it you can feel the liquid. The cat is very nervous if when it is touched, but she eats normally and everything else seems normal. I have been to numerous vets and all of them have no idea. But searcing internet, i am wondering if this can be a problem with salivary glands? Do you have any idea ehat this could be?

Answer
It is possible that there is a salivary cyst that has formed.  This can happen when the salivary duct that carries saliva from the gland into the mouth is damaged.  If there is pus in it, then the cat needs to be on a long term systemic antibiotic based on bacterial culture and antibiotic sensitivity results.