What's the chance my cat has cancer?

You should consult a vet, as s/he is the only one who can answer this question accurately. No one on the internet is going to be able to give you a "good" answer. If, by chance, the cat does have a serious illness, the vet can advise you on all your choices. Ask him what he would do if it was his cat, and consider his answer. My dog had cancer and we were going to remove it, but it turned out that the cancer was in her lungs and other places, so we decided just to keep her comfortable. She lived for about 3 weeks after we found out how bad the cancer was, but she was very happy until about four days before she died. My point is, you need all the information in order to come up with a plan. Talk to the vet. Maybe your cat isn't as sick as you think, and if it is, you can still make informed decisions, as hard as they may be. I have to remind myself all the time not to get too upset about something until I know all the facts. That goes for both my health and the health of my animals. Animals are also very sensitive to people who are stressed or upset, so if you're upset, you're going to make your cat upset, even if there's no reason to be upset.

I'm hoping the vet gave the antibiotic to help the cat from getting secondary infections while you "decide to put the cat down" I have to say though, Covenia has some serious side effects, and I would highly recommend you not see that vet again. It has it's place and is good in certain circumstances where the rewards outweigh the risks, but blindly injecting a cat that they think might have cancer... *sigh* why is it vets don't read ALL the material about medications?? I'm glad you are getting a second opinion. Even if it is cancer, there is a lot that can be done to keep your kitty comfortable. It might even be treatable. I hope you get a positive diagnosis of something that is easily treatable

Antibiotics help just about any sick cat, to a certain degree, it deals with infections. Cancer causes more problems than you'd think. Have a blood test done, if the liver enzymes are through the roof, yes it's too late to help the cat significantly. If they are in the medium range, you can try to get the cat some help. I'd have a second opinion on the cat, from a different clinic. Ask the vet about cardiomyopathy, since the chest and abdomen fill with fluids with that as well. It's not cancer, and some stages of this can be treated.