At this age, diabetes, thyroid problems and renal failure are common. No matter what, you need to see the vet with the cat for some simple bloodwork and possibly for a urinalysis.
At this age, you don't want to take any chances.
For diabetes, there are medications that are effective.
Thyroid problems can be treated.
Renal (kidney) failure responds well to flushing the kidneys and medication.
But any and all of these need to be caught quickly.
I suggest that you see the vet first thing tomorrow for a full exam and possibly bloodwork to find out what's going on.
If finances are a concern for you so I would encourage you to check out care credit, which is a funding source for veterinary bills with easy terms.
Let me know if you still have questions. I would be happy to tell you what I know.