I can imagine how you must feel. I have a 16 year old cat and he also keeps getting bouts of nausea. Each time I think it might be "that time" but he keeps coming out of it. He likes margarine and each time he quits eating I give him a little bit and it seems to help with his nausea and he just starts eating and drinking again. He lost his old buddy (and mine) a while back who was also 16 at the time so every day feels like a bonus now. After you spend around a third of your life with them, it's nearly the same as if they were a relative.
Yes, cancer often causes nausea.
Oh man...that doesn't sound good. I'm in the same boat: I have a couple of old Gentlemen who have shared the bad times with me, and they're getting on in years too