My Mini had a problem with her hind leg. She'd dislocated her hip. It's a rare injury but similar in a way to a broken leg. The vet missed it the first time so the poor thing was in pain (she had some painkillers) for a couple of weeks before an xray which showed the problem, then she had to have an op to break the nodule of bone around her hip joint which then fused a new joint. She's back to how she was before the accident now and can jump three quarters of a metre high if she thinks it'll get her food!
I did some research at the time into broken legs in case that was what it was. Seems bunnies survive very well with only one hind leg. So I wouldn't worry but I would start saving.