My 9 year old golden retriever has been sick on and off for two months, do you know what could be wrong?

I often let her swim in the lake(drainage water) and this time I saw her swallowed an inch diameter something. She was throwing up the next day so I took her to the vet and had an X-ray and blood work-all inconclusive. Put her on a chicken and rice diet and antibiotics for two weeks. As soon as I fed her the regular Iams diet she vomited again. For two more weeks I gave her chicken and tried to ease the Iams back but she doesn't want to eat that. Went back to the vet with a stool sample and that was fine. Now she is on prescription dog food and a little Iams mixed in but she'll only eat if I hand feed her. I don't know if she's drinking enough water. She seems lethargic and depressed. Doesn't even want to go for a walk. No vomiting,no diarrhea. Did the lake water give her a bacterial infection that needs more or different antibiotics(Ampicillin 500mg)? Am I wrong about the lake and it just happens that she got sick after that? Please help.