My apologies that you did not receive an earlier answer. I recently signed on and read your question. Your patience is greatly appreciated!
I'm so sorry your Puss-Puss is not feeling well, today. From your description, it's possible that she has develop an underlying medical condition, there's an infection in her body, she got bitten or stung by an insect or spider that was venomous, and many other possibilities.
Because this is something that is difficult to diagnose without an in person evaluation, it's necessary for her to be seen by a vet or ER vet, immediately. In this way she will be definitely diagnosed and treated appropriately.
Older cats are more prone to develop diabetes, hyperthyroidism, liver and kidney issues, so a routine senior blood panel, including a thyroid (Free T4) test will provide more diagnostic tools for the vet.
I hope all will be well with your Puss-Puss!
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