Help! My cherry barb is sick!

I noticed that one of my cherry barbs is very fat. She was always pretty round, but she is exceptionally round now. I don't know exactly how long she has been this way, but I noticed it yesterday.
This morning she seemed fine, but tonight she wouldn't eat. She seems to be breathing heavier than usual, and its almost like her belly is making it harder for her to swim. She just kind of stays near the bottom of the tank.
I can't see any other visible signs of illness. I checked for pineconing, but saw nothing.
I just did a water change two days ago, so my water should be fine. The other fish are acting normal too, so I don't think it has anything to do with water parameters.
I added sand to the tank a few days ago, and the cherry barbs have been sifting through it. Could she have swallowed some?
Could she be full of eggs? I don't know if that would cause the other symptoms though.
Does anyone know what could be wrong with her, and how I can help her?

She seems to be staying more at the surface of the water now, and she's kind of twitching her body. Can anyone tell me what's wrong? I think she might be dying.

She has been floating in some plants, not moving, for the past half hour. She's deteriorated so quickly, but I'm still not sure what's wrong with her. Can anyone help?

I'm so sorry, I wish I could, but I don't know what's wrong with her
@Fishmanic ? @Crispii ? Any suggestions?

@FishFinatic77 where did the sand come from? All the other fish acting okay? I know you've recently done a water change, but have you done a water test?

I used quickrete sand.
All the other fish are acting fine. I haven't tested my water, but since all the other fish are fine, including my more sensitive honey gourami, I don't think that's the problem.
Can barbs get egg bound? That's what it kind of looks like to me, but I don't know if that's even possible.

She's floating more at the surface again, under some salvinia. She's still doing that weird twitching too.

I think she might be doing better. She's floating in the middle of the tank and sleeping normally. I wonder if she's just constipated? I'm going to fast the fish tomorrow, and then feed some peas. Hopefully that helps her.

Unfortunately, she passed away overnight. I've never seen anything like what she had. She died so quickly.
Is it possible for fish to be eggbound? Or maybe she got too stressed out from the substrate change? She was eating yesterday morning though, and she seemed fine the day before that.
I had her for about 1 1/2 years, and she was fully grown when I bought her. Maybe age had something to do with it?
I just really hope she didn't have anything that my other fish can get.

@Fishmanic, @Crispii, @essjay, @Deanasue, can any of you help me figure out what happened?

I tested my water. Ammonia and nitrites are 0, and nitrates are 5. Like I thought, my water is fine.
I'm still absolutely clueless as to what happened, and I'm paranoid that my other fish are going to get whatever she had. I'm going to keep a very close eye on them for the next few days.

Bump

Bump

Use med for parasite such as Metronidazole and Praziquentel (human med) or just get the parasite med (API general cure) from your LFS.

Add salt and do daily water changes.

Oh I'm so sorry

Barry Tetra said:

Use med for parasite such as Metronidazole and Praziquentel (human med) or just get the parasite med (API general cure) from your LFS.

Add salt and do daily water changes.

Click to expand...

I'm not sure it was a parasite though. Would that kill her so quickly?
I can add some salt today.