help for a little sick canary???

I need help for a little sick canary.

About three weeks ago I went to my local pet shop for some parrot food. I went around looking at the birds and i saw this canary on the bottom of the cage. Her vent was very red and swollen. I thought she was egg bound so I called the manager. She took the canary away to the next door pet clinic. I phoned three times to find out how the little canary was doing but had no luck.

Yesterday I went back to the same pet shop for some pet food and asked the manager about the canary. She told me that she was back in the main cage as she was better. I foound her at the bottom of the cage. The other canaries were pecking at her and would not let her go to the perches. She looked very stressed out and had lots of feather missing so I bought her straightaway. Her vent is still a bit swollen but the skin looks a normal colour. It doesn't seem to be inflammed anymore.

She has slight diahrrea but she has been eating well. What should I do? I don't want to give her back to the shop. I think she is just too stressed out. I put some grit on the floor of the cage. Any help would be greatly appreciated. thank you:o)