My 17yr old cat(Sammy)had the same symptoms,from an infected tooth.The tooth was pulled,treated with antibiotics.He did not too bad for about a year,then again the same symptoms,more antibiotics until one day he was sneezing blood and coughing blood,only then was cancer suspected and unfortunately too late to do anything about.He passed away at 18yrs.
I am not saying that your cat has cancer,but bad teeth can cause all kinds of problem,I've had teeth extracted on cats a few times.
Tartar on teeth can cause the gums to get infected and it's important to have the cats teeth cleaned.
"The cruelest animal is the Human animal"
3 kitties,Rocky(r.i.p my boy),Chico,Vinnie