Hmm, Michelle it looks and sounds like your doing all the right things.
As i was reading your post you covered everything i was thinking it could be. You gave very good details but it still is hard to say what it could be without seeing her.
Maybe it is age like you mentioned... could be some type of tumor?? As long as she is acting normally and eating when you feed her try not to worry too much.
I would suggest feeding her every other day. I would also forgo any pellets for now. She will be fine that way and maybe give her some extra time to evacuate her little belly.
My friend who breeds and raises bettas has had some in the past that had swellings and then got a little twisted looking as they got older but they did fine, acted normal and were happy, lived for quite a long time. Some even developed a tumor but continued to act normally.They eventually died of old age.
She has also had some that had tumors or had the diseases you covered but then didnt fair very well even with treatment and passed away soon afterward. Some did respond to treatment. Of course lotz depends on how quickly it is caught and treated.
Just keep an eye on your Fishies. I know your taking great care of them.
Maybe the swelling will start to diminish soon or maybe Kanya will just continue be that way as she gets older. Or maybe you will figure out if it is a some type of condition that you will be able to treat successfully.
Old age stinks, doesnt it!?
We and even our pets start to fall apart. BLAH!!
I hope Kanya improves and remains a happy fishie.