Blood work was fine but the cat isn't. Stomach issues and cancer questions

My cat Jade is nearing 16 years old, and she's always been happy and playful and lively. Thursday I ended up taking her to the vet because suddenly something was very, very wrong. She had no life to her, basically. She was limp and wouldn't look at me when I called her and just didn't care about anything. She normally is very wonderful and annoying in the morning, and it took me fifteen minutes to even find where she was hiding.

When I finally got her to the vet, she still just was barely doing anything, not moving, breathing veryyyy slowly, and it was terrifying to see her like that. The vet did some blood work and gave her a quick little check up, looking at her eye and gums and taking her temperature and stuff. The blood work came back fine, and actually the vet said she was really impressed with her health. Because of this, she said that there's really two possible solutions as to why Jade isn't feeling well.

The first is that she's caught some sort of bug. She's always had a pretty sensitive stomach and thrown up easily, so it could be something to do with that. They gave her a steroid shot and an antibiotic shot that will last her a couple of weeks so hopefully she'll get better. They gave her some fluids under her skin because she was dehydrated as well. Of course, I was told that the second possibility is that she's got cancer, and with her history of throwing up every day or so, I'm pretty worried.

Yesterday, Friday, she was mostly unresponsive again. I did get a little bit of activity out of her during the evening, and she was waving her tail slowly and walked around a little after I got her up to see how she was doing. She ate a little piece of turkey too.

This morning, she looks like she's feeling a little better but is still not all there. I gave her more turkey and she walked around just a little bit. What I'm noticing now though is that she's barely using her back legs. She's walking, but it looks like she's going through a LOT of effort to do so. Her hips are caved in and she's really struggling to move.

Because of how bad she was the other day and just because I know she's an older cat, I'm just worried for my cat. I love her so much, and I never want her to suffer. Does her walking sound concerning or do you think it's something that should clear up with the medicine she was given? I'm just so scared that she might have cancer, and if she does, I just want to make sure the quality of her life is wonderful and don't want to prolong any suffering. I've had her for 15 years and she's my best friend, so I want to make the most educated decision that I possibly can.

The vet told me to call back on Monday, so I have a couple of days to keep an eye on her and assess how she's feeling.Of course I don't want to make a decision too soon, but honestly after seeing how much trouble she's having right now, I'd rather make a decision too soon and have her happy and in no more pain than prolonging any hurting.

Does anyone have any sort of advice as to how cancer for cats works or if there's a way I can tell that it might just be a bug that's hitting her really hard?

You're in a tough spot right now and I agree, you don't want  your girl to suffer longer than she has to.   I'm not sure there is any one way that cats behave when they have cancer - it kind of depends on the specific cancer involved.  My dear old boy Milo died at age 14 from oral cancer and his symptoms were quite different than the ones you describe.  He basically couldn't eat, despite wanting to, because it hurt so much.  He did hide at first and was definitely lethargic but did not have any problems walking.  

Did your vet mention what kind of cancer (s)he suspected?  And did the vet suggest doing an ultrasound to narrow it down?   I would think that's the first thing to do before worrying too much.  Of course, that's easier said than done and you are naturally very worried about your friend.  Sixteen is getting up there in years though so it might just be that if Jade simply has a bad infection, it is taking her longer to shake off than it would if she were younger.   I hope she gets better.  It is so hard - they can't tell us what is wrong and so we have to guess.  Meanwhile give her as much love and attention as you can.  Milo really responded to that and it helped me as well. 

Since she is older, we all decided that it didn't make too much sense to do all of the diagnostic work and spend the money and have her too stressed out about all of it. She was basically on what seemed like her last day when I brought her in.

I'm watching her today and she's gotten up once this afternoon, made a small walk around, then went and hid back under the bed again. She's still not walking well.

I guess it's just things I need to bring up to the vet Monday. At least, it doesn't seem like I have to worry too much right now, because she's not acting like she's in pain or anything when I do pull her out from under the bed to check on her. She meows a little because I don't think she's happy with me moving her from her resting spot, but I just want to make sure she's doing okay.

Also her eyes are very dull today. There's no shine to them whatsoever.