D.W.
Is the litter box upstairs too? If not, she may not be able to make it there anymore. Was the kitty litter changed recently (new brand?). If so, that may be an issue.
Did you change food on her? Or, are you using the cheaper store brands? This could cause issues with her digestive system. The cheaper brands, such as Friskies (which is what my cat had), will change the formula in the food so that they can keep the price consistent. They won't change the packaging or anything, so you will never know. But your cat will. More expensive foods like Iams, Eukanuba, and Science Diet will increase the price of the food to keep the formula consistent. Nothing wrong with the cheaper brands, but sometimes as cats get older, they become more sensitive to such changes.
At any rate, it certainly would not hurt to take her to the vet for a quick checkup.