If her scales are sticking out and she looks like a pinecone, she has an internal bacterial infection called 'dropsy'. This is serious, and often fatal even with antibacterial treatment.
If not, she could be constipated. Adding 1tsp/10g epsom salts to the tank won't hurt your other fish (I have no idea if it has a negative effect on invertibrates or plants though, so if you have either please do some research) and could help her pass the blockage. I would fast your fish for a few days; this won't hurt any of them, and could save the barb's life.
You said she's still eating, you could try feeding her a thawed out frozen pea (remove the shell, just feed the inside), which acts like a laxative for fish. Don't feed anything else for the next 48-72 hours though, if it's a blockage feeding her will make it worse.
If you have the option, it wouldn't hurt to put her in a hospital tank to be treated and recover away from her tankmates.
Good luck, keep us updated.