Good luck at the vet's; I hope you can get her there soon, it is never good to wait on elderly animals with health problems.
I should like to mention that OTC wormers are NOT always safe or effective. It it best to talk to your vet prior to buying them. This way, if you can't afford a vet-perscribed dewormer, they're better than nothing. Using a more valid dewormer perscribed by a vet, however, is best. Honestly, I wouldn't trust OTC products on older animals, since they often times have sensitive digestive tracts as it is.
It is very dangerous to self-diagnose your animals. Cats at that age are likely candidates for thyroid problems, diabetes, and other conditions that could be eased with medications. However, because geriatric animals are so very delicate, it is of vital importance that health conditions be treated promptly and properly. Even kidney failure, the number one killer of cats, can be slowed and made less unpleasant for the cat with fluids and modified diet.
Question - is you cat on any preventatives? Tapeworm is often spread by external parasites, as is heartworm. Your cat should be on something like Revolution all year 'round to prevent fleas, ticks, heartworm, and some internal parasites, and if she is allowed outdoors, she should be dewormed regularly. Free-roaming outdoor cats need much more protection in the form of vaccines, preventatives, and deworming, since they are being expose to a much more dangerous and disease ridden environment than the house, or even an enclosure. Every bird and mouse they chomp on is full of worms. Every cat they fight with is full of diseases. Every brushy field they walk through is loaded with ticks. So, if your old gal isn't on preventatives, and turns out to have something easily treatable, I'd talk to your vet about them so she doesn't get ill again.
