I tend to follow the rule of never panic... Rats do really strange things sometimes... then in a day or two they return to normal... If you don't see an apparent health issue give it a little time.
Cloud spent a day walking on three legs, I didn't see anything wrong and she was back to normal the next day. I suppose something hurt her, maybe from a fall... who knows? I'd guess that Zelda might have some discomfort she's compensating for...
Naturally if things get more abnormal or this doesn't resolve itself a vet visit might be in order, if you have a good rat vet... It might also be worth a post under the rat health section.
Hope Zelda feels better soon.