Sheep even when an infection in the foot tend to try and put a little weight on it , if shes holding it up then it might suggest damage to the leg eg knee ,shoulder . For her not to go with the others suggests pain , even a very lame sheep trys to keep up with the flock it is in her DNA not to be seen as weak . As Anke says at this point in her gestation she can go down with twin lamb VERY quickly , I would have brought her inside once she separated her self to reduce stress and keep her warm and maybe give her calcium borogluconate to give a boost , it would do no harm . Please keep a close eye on her tommorow