Hi
Our 18yr old cat who had previously been very skinny and had been diagnosed with hyperthyroidism and a heart murmer a couple of years ago, suddenly developed a very swollen tummy a couple of weeks ago. He wasn't in pain and was eating, drinking, urinating and defecating appropriately and enjoying sitting in the autumn sunshine right up til Tuesday of this week.
Without going into distressing details, our beloved old man passed away on Wednesday and is now buried in the bank where he used to sunbathe.
From what I can gather from this forum, he was possibly suffering from a tumour on an organ or the hyperthyroidism had lead to fluid build up in his tummy. However, at 18 we were reluctant to commence a course of distressing treatment for him as he has always been a very reluctant visitor to the vets on the rare occasions he's had to go. And he didn't seem to be suffering as his appetite and bodily functions were unaffected and he wasn't in pain.
However, your Nala is younger and more likely to benefit from treatment.
I hope all goes well.