sick rat - stomach upset?

Question
This evening my rat began showing serious signs of illness. She is visibly uncomfortable--stretching and changing positions often. I've coaxed her to walk about the cage briefly and she was dragging her lower abdomen, especially on one side. It's subtle, but it also appears that her belly is bloated. I've only been able to get her to drink a few sips of water. The only thing different in her diet today is half of a cherry tomato from my garden (well washed). She is approximately three years old and has always been in excellent health--very active, healthy appetite, appropriate weight, thick coat. Please help.

Answer
 It could be a number of things such as a tumor or internal abscess, pyometra, bowel obstruction, or even a pregnancy (though since you don't have a male that is unlikely).  Whatever the case, it sounds like she is in pain and it may be something serious.  I would get her to the vet as soon as you can; hopefully whatever is wrong can be taken care of easily before it is too late.  I hope she feels better soon, good luck!