My Rat Has Unknown weightloss and Habit changes....

Question
Hello, I was coming to ask a question about my rat, Dumbo, named for her breed, the Dumbo Earred Rat. I had gotten her from a Petsmart about a year ago now, and she was healthy, lively, and shy. She's never been the type to come out of her cage, and she's not one to like being handled, so I usually pet her in her cage, a glass aquarium with a screen top. This past month or so, I've noticed a drastic weight loss and she's been breathing very very heavily, almost labored, especially whenever I pet her or something gets her going. I try giving her treats, even pop tarts and stuff she should be happy to take, and she turns away from them all and doesnt eat anything I give her. As well as loss of appetite, she's lethargic and doesn't like to move around much anymore. if i open the cage she'll come out sometimes and just lay on top of her wooden hut and just sit, breathing hard. I feed them daily with just a basic grain diet. usually not always the same, I switch between a few select brands I've used in the past. Her tank mate, also female, is an ougoing and fun rat, and her health has not changed at all with Dumbo's. I'm extremelly concerned. I also read another article about a rat losing weight but she still ate and had her same health otherwise, I did the tummy feel thing, no lumps or squeaks as well as I could do it. I couldnt get her on her back without her freaking out and making her breathing worse, so I let her stand on her two back feet and felt. Her teeth look ok, and her eyes look ok. I can't explain what could cause her condition. Could you Please help me!?

Answer
Generally these are the first symptoms that display with the onset of pneumonia, a basic upper respiratory infection, or congestive heart failure. There's not much to do to diagnose one or the other without a visit to the vet; and all three are very treatable if caught early enough. The only way to determine one from the other is to listen to her breathing by holding her chest to your ear. If you hear chest congestion, rattling; it's likely something in the lungs (like the aforementioned pneumonia or URI).

Wish I could help more! Good luck.