My cat has been acting weird (long post)

Hello everyone; thank you for your time, and let me say that I am in no way trying to replace an actual visit to the vet with Reddit advice. I ask only for your thoughts on this matter and if you've dealt with anything similar, or if you even know what this is...

So around April 23 my cat, Yuki, was acting unusually quiet. Mind you she is quite reserved, but very food motivated and clingy; on the 23rd she was very distant and did not care for her favorite wet food and snacks at all. She also did not drink water, nor did she use the litterbox. She did not seem to be in any obvious pain or discomfort, but she did seem sluggish. She did not react to my touch and I tried touching the spots it seemed she was hiding, to no reaction. Since this was late at night, and over here our veterinary charges 70€ upfront for an emergency appointment, I was skeptical, and the lady on the phone informed me how, based on my description of things, "it was fine" if I waited until the 24th, but that I had to bring kitty in if she hadn't used the litterbox come morning. So I scooped Yuki up and we went to sleep.

Morning comes and Yuki hadn't even moved from my side, to which I took her to the vet. She stayed there until the 25th, where I was told that they couldn't find anything wrong with Yuki's diagnosis and that everything they ran came out fine. In the meantime Yuki had used the litterbox multiple times, but had not eaten and had not drinken voluntarily, instead being hydrated through a serum. I was given the option of taking her home and "hoping that she ate anything in the next 48h" or leave her with them some more time, which even though I wanted to, I quite literally would be left with 11 cents in my bank account after paying what they were already charging me.

So I took Yuki home and she fortunately showed interest in her food; she took very small bites of her wet food and was quite interested in snacks. She also drank a miniscule amount of water and used the litterbox. The coming days would see Yuki getting even better, to the point of me believing this was just some random symptom she never felt before after her being in heat shortly before this incident.

Yesterday, however, May 6th, Yuki again hid herself under the bed and would not come out for anything. At night, around 10 or so hours after her hiding under the bed, she came out for food, water, the litterbox and, shortly after, demanding attention, while walking a bit slowly. Today, she did it again from 12 PM to 2 PM, where she came out to take care of her needs as usual, and to sit on my lap while I work. At around 3 something PM, I had to get up, picking Yuki up and placing her on the ground - she did not resist or complain, but her body seemed tense. I then thought "wait, you can just wait for me in the chair instead of the ground", and went to pick her up. Yuki likes it when I pick her up in a specific way: I place my hand and arm under her belly, and I place the other hand under her chest, lifting her up gently; when I went to do this, she yelped quite loudly before I lifted her up, to which I immediately backed off and saw as she very slowly hid under the bed again...

I am now writing this post after what just happened, and would appreciate any advice. I dread going back to the vet both because Yuki, who is very gentle, grew to be very aggressive while in the vet (and only there), and, if that weren't enough, I am likely to be penniless again if I were to go back, leaving both me AND Yuki hungry for the month. It is an extremely shitty situation and since I can type this post out during work hours, while a vet call will need to be done during lunch break, this will have to do for now.