Help! I think our gerbil is sick!

[Nancy BisCamp]

We have had a female gerbil for over 2 years now. She is about 2 1/2 years old. She has always been healthy, however, the past 3 days she is not looking well. She looks all wet underneath and she stands all the time with her rear end up and front legs resting. She is breathing rapidly. She may even be swelled up at her backside. I don't see any evidence of diarrhea. She is eating and drinking still, but I am worried she could be dying soon. Please help if you have any advice of what we can do to make her more comfortable or information as to what could be wrong with her. Thanks, Nancy