Update: Recovering (?) Anorexic Cat

This post is a supplement to this one, which is essential reading: https://www.reddit.com/r/AskVet/s/gqarEf8fY7

I made that post 5 days ago, fairly optimistic about her recovery. I am visiting again today after being in close touch with my family, and despite her slowly eating more and more with each passing day, even reaching triple digits the last 2, today her appetite seems to have crashed. She ate maybe half a can early this morning and hasn’t touched anything since. I noticed last Thursday her breathing seemed a little fast and today I counted 50, then later 36 breaths per minute. She rejected attempts at hand feeding and the smell of her trout fancy feast seemed to itself make her nauseous. She is alert and responsive but only wants to lay down wherever she is. She appears to have lost .7 lbs from last week, 5.5 to 4.8, despite her improved eating habits and lack of activity. This could be due to her inappetite today. I’d estimate that she’s had 50 calories on the high end today, and ate about 120 yesterday. I know this is a vet visit, but my dad already spent $600 on her first vet visit and likely won’t keep spending much more to find the cause as she is already 12. Does anyone have ideas as to what the culprit might be?