She could have subclinical hypocalcaemia. I would keep a close eye and consider treating for twin lamb disease and with calcium as a precaution.
This!
We have had several do this, this year they initially look a bit lame (nothing wrong with the feet) they then go down and struggle to get up, had 9 do it on day 2 after lambing, after a good dose of calcium solution under the skin and some energy drink (twin lamb gear or similar) they were up and about as if nothing had happened. If you don't treat it they die in most situations. Lack of Calcium this year has been a major issue in our area with several herds of suckler cows suffering where they have never suffered before.
It could be because of the attrocious grass growing conditions last year that meant the summer grazing had little value in it plus also the silage/hay fed over winter is essentially the same deficient grass.
All mine had adlib mineral licks and energy licks as well as 500gm's/ day cake and adlib hay.
The real test will be when the main flock start lambing in April (i hope it won't be similar) as we had 9 out of 21 do it, the main flock is 750