How long ago was she spayed and did the problem happen after or before she got spayed?
Food/Eating
If vet confirmed that she lost weight, then obviously there is an issue here.
Some cats have a slow metabolism which means they dont need to eat too much, but yours does need to eat more.
This is why the Vet asked to give it more time. If she continues to lose more weight, then they might try something. This will of course be costly.
So you need to get her eating - this is your first objective.
You will need to try out different foods because she is clearly a fussy eater and you simply cannot force-feed her - this doesnt work.
Have you tried out Whiskas wet food?
Go down to your supermarket and buy 1 pack of each food that you havent tried out yet.
See if she likes any of these.
Also, pop over to zooplus and get a variety of small packs of dry food.
My cats like Applaws (dry cat food) a lot and they dislike virtually all other brands of dry cat food.
You could get a 400g pack of that. Get chicken flavour and the kitten flavour. Applaws has the highest concentration of protein of any dry cat food on the market, and cats can get fat eating this (mine did a few years back, so I temporarily stopped - now I "dose" the food carefully in a food ball). Your cat will need concentrated calories if she has a small stomach.
Faeces
Bear in mind that when you find a food that she is prepared to eat, she may do watery poos, as you are changing the diet. But this is ok. Our main objective is to get her eating. In time the watery poos will stop. If not, the vet can give you something to firm up the poos.
Meowing, Screaming, Shouting
Do you have toys and does she chase the toys until she is panting?
Do you get rough with her and simulate a cat play fight - does she fight with your hand?
Is she "ok" with you handling her, any time that you see fit?
Does she know you are the boss?
Are you in full control of her?
Is she scared of you?
Are you scared of her?
Does she ever come and sit in your lap of her own accord?
Does she ever come to you and rub her body against your leg, of her own accord?
Are you both comfortable enough for you to gently bite or kiss her cheek, while holding her, with her being completely relaxed with this behaviour? If this last point is true, it means that you are completely free with eachother and there is 100% trust, in both directions.
Some people say that if a cat rolls on its back that it trusts you - I disagree with this. For example, my sister's cat occasionally rolls on its back for me, but as I approach I always sense that he is weary of me and shows this by getting on his feet.
Most owners dont understand this but you must get the message through to the cat that you "the boss" and that while you are in the room (at least), you are in full control of the environment and she has nothing to fear. That you can handle her any time you want to, but when you do, he/she is safe. Your own gut instinct will tell you when your cat trusts you 100%.
Do you have any other people living with you?
Is she scared of other people?
When was the last time she physically touched another cat?
Is she an indoor cat?
Where does the cat sleep - does she have own bed?
Does she sleep in the same room as you?
I'm asking all these questions, as I am trying to zero in for the reason she is screaming/shouting for no apparent reason. My gut tells me that she wants attention. If she doesnt have a playmate, then its possible she wants to play.