Pet Rat - Weird Symptoms - What's Wrong?

My boyfriend has a pet rat named Agatha, and she is less than a year old. However, she has been acting strange and we just noticed it, but have never even seen or heard of these symptoms. I can only describe it.

She walks forward, sucks in her stomach at the waist, to the point where she looks emaciated, and this makes it hard for her to balance as she keeps walking, so her hind legs drag a little. He says it looks as though she's trying to turn by standing with both legs, limp, in one spot. I think it looks as though she tries to bend herself in half like a hose.

We looked everywhere for symptoms list, and could not identify what was going on. She still eats, is active, playful, but does a look a little tired. The problem just started today.

If anybody knows what's wrong with Agatha, please help!