Hi there,
If there are no obvious injuries like a break or a knock, or an injury to her back, sore feet - scald ,broken or overlong nails can make them very sore seemingly overnight. Scald can be fixed with antibiotic tetravet (?) blue spray. Scald looks like a paper cut between the two parts of the hoof. Also mud gone hard can make their feet sore.
If it is her joints antibiotics are worth a go, you may need to talk to your vet. I have used ampicillan la (long acting)injection here in Nz for what i think you call joint ill and after a few days it gets significantly better. Ussually you have to do it a couple times. This would also help scald. One of my sheep broke a nail somehow yesterday, and she is very sore, i trimmed it so it wouldnt break further, she doesnt want to walk around.
If her feet and legs are fine, check she is not bloated as their legs can swell and be very sore with that - but i assume other then difficulty getting up, that she is fine?
I have a large lamb who was injured by some other lambs when she was about 4 months. She was lifting her back legs looking like joint ill but turned out to be a back injury. The vet gave her anti inflam i think, or steroid inj cant remember exactly what, it helped a bit but she has a permanent odd walk.
Theres a few possibilities! Good luck. I hope it is just her hooves needing a trim
