I would say that it's not unheard of but it's not common, no.
We do get the odd lamb each year that is less active; we would normally give a good vitamin shot and usually that sorts them out.
It could be that she's not really getting enough milk, so hasn't the energy her siblings do - could you top her up?
What did you find out about shivering after feeding? I only know they'll shiver if their milk was cold, but when they're feeding off mum that won't be the case.