please help

So my cat is about 15/16 years old and for the past month she's been struggling with vomiting. As long as i've had her (2011) she's been routinely vomiting hair balls every so often. Since about the beginning of September she has become lethargic, doesn't want to eat (she would eat about half her food, drink her water) and was just very uncomfortable. Even after walking she would breathe heavy as if she wants to vomit but anytime she tried she wouldn't. if u press on her stomach she is not in any kinds of pain at all shes just purrs uncomfortably.

I took her to the vet two weeks ago, she (my cat) had a fit where she was just under distress and they had to put her on oxygen, after that they did bloodwork and urine and everything came back normal save for a little elevated count that is pre kidney disease but the vet said its normal at my cats age and my vet gave her a shot to stop the nausea and some nausea pills for 8 days.

After the vet, my gosh it was like seeing a whole different cat within a day my cat vomited (it wasn't a hair ball) but she ate and was active again, it was like this for about a two weeks.... She's back to being lethargic, nauseous except this time she doesn't want to eat at all. She just lays in bed and only gets up to use the bathroom and drink water. I've tried catlax, those drops to help with stomach aches, we got her new food I just don't know what to do anymore, she's super uncomfortable and gagging, i know she's hungry bc she wants to eat and goes to her food bowl eats a bit and then starts gagging. I dont like looking at her struggle like this idk what to do.

so I guess from all of this has anyone ever dealt with a gagging cat who really has nothing wrong with them? any tips or advice?