My platy doesn't eat

I don't know what to do.
It's my female platy. She hasn't been interested in food for 5 days now. She shows not other symptoms, excepts she looks weakened by the day.
She used to be one the bigger eaters in my tank and now... nothing. I'm very worried. All the other fish are fine. Flakes and dried bloodworms pass right next to her and she just won't eat! It's like she's letting herself die or something.
I don't think she's pregnant either. She doesn't look round.
Levels: ammo 0, nitrite 0, nitrate 15 ish.

Please help!
Thanks for the advice

Thinking back, what does it look like when this fish goes to the bathroom? Also, does it have a red, inflamed anus?

Hi, thanks for the quick reply.
Well, since she hasn't been eating, she hasn't gone to the bathroom either! Before this situation her poop was dark brown, pinkish sometimes, and no, she doesn't have an inflamed anus. She's looking thinner though. Last night I noticed she had some difficulty swimming but I thought it was due to lack of food in her system.
Any other ideas?

Oh, by the way I did my weekly waterchange on Monday.

Not eating is a hard problem to diagnose. Since she doen't have a red anus or stringy, white poo, i would hesitate to say it is internal parasites. Getting thinner can also be a hard one, especially since she hasn't been eating.

This is a total random long shot,. but when you look down on her from the top, is her spine bent at all, like a S curve?

Hi again,
Nope, no curved spine either. I notice her ¨lower lip¨, if we can call it that¨, moving a little faster. I even thought there might be something stuck in her mouth that prevents her phisically from eating but I don´t see anything. It´s really dificult to look inside her mouth anyway.
I´m really at my wits end. Yesterday she tried to eat ONE little flake, but spat it right back.

Please guys feel free to contribute!
Thanks for the help!

Poor lil thing. Is she being bullied? Sometimes bullying will make them feel so stressed they stop eating. Can you seperate her, even in a breeders net for a few days? Make sure she gets a few crushed flake or a defrosted softened pea to try (in case she's constipated). Best of luck.
Hugs,
P.

It's more like she's the bully! She wouldn't let the male platy get fresh with her if you know what I mean. But you're right, she might have gotten tired. I'll put her in the maternity box for a few days. I hope this works.
I'll let know. Thanks

Update:

Tonight I noticed something. She has some sort of bump in her belly. It's not visible from the sides but from the front and rear view. It looks larger from the right side, and part of her back protrudes from the water surface. Here are some images. She has some difficulty swimming and still not interested in food. I've put her inside the maternity box and she wasn't too pleased about it!
I put some extra salt and stresscoat. I have some Aquarium Pharmaceuticals Furan-2 capsules, but since I'm nto sure what she's got I don't know if they are indicated or not.
I hope someone could help. I'm very sad for her.
I'll appreciate any advice.