Rat limping!!

I just went to take out my rat Olivia, and noticed she was limping. Her left hind leg is drawn up against her side, and she is squeaking more than normal. She doesn't look like she is in much pain, but is having some difficulty getting around. She was not limping yesterday, so I'm guessing something happened in the night. This is a little scary, because she has never had any problems before. What should I do??