Betta fish bloated on a side: what is this!?

[16mirta]

I have a female Betta Splenders, she has bright colors, no issues swimming but she has a really bad growth on a side of the belly.
We quarantened her in a 5L tank to check how her feces were and to make her and other fishes safe.
At the beginning she had stringy poop and we (my bf and I) thought it might be bloated and costipated.
We made her some epsom salt baths and we noticed a change on the color and shape. We starved her for three days and then tried to give pealed boiled peaces of pea. Then we gave her (instead of dry pellets with might have made her constipated) some creamy brine shrimps.
But she was still bloated, then we noticed the poop, which at the beginning was red from pellets, became bigger and white/yellowish like mucus. After a while it changed and started to have some black spots in the poop, not stringy anymore.
The problem is she was still bloated and she's still. I started to think it was intestinal parasites and introduced in her diet some garlic. Unfortunately in Italy we don't have really specific medicines for those, so I bought eSHA-ndx and started yesterday the treatment. I don't think it will help the problem.
I even thought it could have been unabsorbed eggs but it don't think it would be only on one side.
I checked her from the top to see if she resambled a pinecone but she doesn't, if not only on the stretched spot.
I noticed she has vertivally curved spine and that made me think again of dropsy, but my boyfriend told me she's always been like that.
I'll let here some pictures of the quarantine tank.
She has a heater, we change the water and check the parameters many times but we are wondering if a filter might help. Right now we put her an oxigenator to help her breathe, but not too strong to not stress her out. We always have catappa leaves inside for bacteria and try to not leave her too much at the light.
Maybe we should add some fake plants for her to hide or higher the amount of water inside the tank but we let it lower for her to be able to breathe from the surfaces if she needs too.
Her colors are bright, she enjoys garlic+brine shrimp, actually she even jumps to catch it out of water, but after one time, she's full already.
The eyes seem normal.
Really, we can't understand what to do, and actually tried a microsurgery to help her but I think it didn't change much and not knowing too well the fish anatomy we didn't push it too far (She's pretty fine even after this). We don't know how to help her and we want her to be ok.
I even thought it mught be an inner tumor. I don't know, we really need help!
Some pictures are from a month ago (the last four).
Might it be kidney failure?!
Please, help.

 

[Colin_T]

I would say it's either a tumour or cyst growing inside her. There's no cure for either so just keep her comfortable and when she stops eating or has trouble swimming, then euthanise her.

 

[16mirta]

I would say it's either a tumour or cyst growing inside her. There's no cure for either so just keep her comfortable and when she stops eating or has trouble swimming, then euthanise her.

I was thinking the same, but looking at the feces, I'm not so sure (she's eating garlic+brine shrimps).
The black part was harder than the rest. What do you think?